Hmmmm.
OK.
Assuming we change from around 1 jan 1935.
Industry. Britain was actually coming out of the Depression very well by now, BUT... The recovery was heavily biased to new light industry and infrastructure in the South, especially around London. Now a big prolem was the fanatical insistence by the Bank of England and the Treasury of a 'balanced budget' and no inflation. But there had been DEFLATION for quite a few years, which had made the issue of interest repayments far worse. Now off the gold standard, interest rates were dropping fast, what they need to realise first is that SOME inflation (at least enough to cover off the previous deflation) would be a good thing, and borrow some money to help the economy.
Target this money in the north, to improve, re-equip and help the older industries - steel, coal, shipbuilding, heavy industry, and place some additional RN orders to help. It will reduce unemployment (recovering some of the money being used). You could also improve infrastructure, again meaning more orders for steel. Build new plant, but keep using the old (Britain didn't make enough steel for itself, a first target would be to at least make that amount). A shortage of heavy industrial production and steel was a limiting factor when they tried to ramp up arms production later on.
The RN was actually quite aware of the advantages of welding. They had an all-welded sloop, and Ark Royal was 60% welded. They need to be tough on forcing acceptance (use the industrial money as a lever to bully the unions with). Unlike certain navies (I'm looking at you, KM!) their welding was good, bits of ships didn't break off.
Realise that the Japanese (the real naval threat in 1935) had left the treaty system, and either now ignore it, or at least make one with the USA-France-UK that gives these navies what they need.
Lay down the KGV class, with 9x15" guns, on 1/1/35. That gets 4-5 in service for Sept 39.
Lay down two more Ark Royal class (to alleviate unemployment).
Force the RAF to give up the FAA, and give the navy that budget back, and allow the FAA to supply the aircraft for maritime patrol. The RAF were supposed to do this, they used much of the money to build useless bombers instead. Patrol planes in 1939 remove most of the need to use fleet carriers for cover, so no loss of the Courageous.
Have the FAA order some decent aircraft. Either pay Bristol to build a 1200hp radial (not hard, it was proposed) to power them. The RAF never liked the Battle, but a version with a shorter wingspan (same area) would make a very good torpedo/bomb/recon plane. It was also stresses for an 80 degree dive, there's your dive bomber sorted. There were planes being developed that would make a good naval fighter (with some mods), but with no urgency as, again, RAF.
The FAA would have a lot more interest in heavier armament, they had a limited time to shoot a big plane down (interception of things like the Condor was a big need), pushing the 20mm cannon along.
If the FAA controls Coastal Commend, someone might actually ask to test the bombs, and do exercises against subs (something the RAF seemed to think was beneath them)
The armoured carrier (no, it wasn't an armoured deck!) was actually a marginal decision. If better fighters were coming, the equation swings more towards more fighters to stop a raid rather than trying to survive it, so keep on building Ark Royal derivatived. Without a new treaty, they'd probably come out at 25,000t or better rather than try and cram a design into a lesser displacement. That allows you 60-70 planes and decent armour protection (even ignoring the deck armour, the Illustrious class was TOUGH)
Light/trade protection had been a RN interest for decades. You do have some slips available (they are roughly cruiser-length, but there are limitations of equipment for cruisers, so lay a few down instead)
Build more destroyers. 3 flotillas a year is doable (just), either Tribal or J-class. You need to replace a lot of old, small destroyers, so push this as 'just replacements' and 'supporting the unemployed shipbuilders'. With more destroyers, you can convert the V/W's into escorts a lot sooner.
A/S ships. There is a problem building a lot of these early, manning them. They were expected to be build quickly, so build say 6 a year for a couple of years to experimental designs. Look for simplicity and speed of build, the use of diesels (for the long range versions). Then its a lot easier to start spamming them out.
A/S weapons. The RN had experimented with forward thrown deth charges, but it failed due to the misconception of using a single long range one that needed an accuracy that wasny doable at the time. have someone look at options (yes, this needs some money), and something like hedgehog is easy for 1939, and pretty easy to refit. The star-shell is known to be rubbish, why not develop Snowflake early? It's not rocekt science...well, actuall it is, but...!
The RN WAS looking at convoys. Thats why they started a few days before the war. What they didnt do was develop an escort group to go around the convoy. Again, takes some money, but with 4 years you can make those escorts a lot better, and avoid some of the early mistakes.
The RAF needs to be taken out behind the bike sheds and beaten until they show sense on bombers. As in, at least given them a decent bomb sight and navigation aids. And prioritise the fighters for the Merins.
Have the ramp-head Merlin die an early and unlamented death. That gives you 12-18 months lead on the later marks of the engine, more hp to help your planes. Instead of trying to develop loads of engines, show some sence. The Merlin and Griffon are all RR need to develop. Bristol can quite easily build a 1200hp radial, and the Hercules issues should have been sorted WAY earlier.
The Army suffered a lot from the decision that it wouldn't be sen to Europe. Admit it will be, that gives you 4 years to equip it. Some weapons development was leisurely, like the 25pdr and the Bren, speed this up. Design a decent tank for Europe, something around 18-20 tons, and insist it doesnt have to fire on the move. Something like a bit heavier, better Valentine would be great for 1939.
Go for a core of 6 Lorried and an Armoured Division as the core, then think on the territorials. Some short term service (again to reduce unemployment), say a 2-3 year term, would give a pool of young infantry to help build up the older men serving in the territorial units. If your going to be in Europe, some decent armoured cars would be needed.
Give the Army back the close support squadrons the Experimental armour used to train with. A 20mm armed Hurricane would be very suitable as a start. As would some 20mm guns to defend with.
None of these changes really requires magic (well, making the RAF see sence might)
OK.
Assuming we change from around 1 jan 1935.
Industry. Britain was actually coming out of the Depression very well by now, BUT... The recovery was heavily biased to new light industry and infrastructure in the South, especially around London. Now a big prolem was the fanatical insistence by the Bank of England and the Treasury of a 'balanced budget' and no inflation. But there had been DEFLATION for quite a few years, which had made the issue of interest repayments far worse. Now off the gold standard, interest rates were dropping fast, what they need to realise first is that SOME inflation (at least enough to cover off the previous deflation) would be a good thing, and borrow some money to help the economy.
Target this money in the north, to improve, re-equip and help the older industries - steel, coal, shipbuilding, heavy industry, and place some additional RN orders to help. It will reduce unemployment (recovering some of the money being used). You could also improve infrastructure, again meaning more orders for steel. Build new plant, but keep using the old (Britain didn't make enough steel for itself, a first target would be to at least make that amount). A shortage of heavy industrial production and steel was a limiting factor when they tried to ramp up arms production later on.
The RN was actually quite aware of the advantages of welding. They had an all-welded sloop, and Ark Royal was 60% welded. They need to be tough on forcing acceptance (use the industrial money as a lever to bully the unions with). Unlike certain navies (I'm looking at you, KM!) their welding was good, bits of ships didn't break off.
Realise that the Japanese (the real naval threat in 1935) had left the treaty system, and either now ignore it, or at least make one with the USA-France-UK that gives these navies what they need.
Lay down the KGV class, with 9x15" guns, on 1/1/35. That gets 4-5 in service for Sept 39.
Lay down two more Ark Royal class (to alleviate unemployment).
Force the RAF to give up the FAA, and give the navy that budget back, and allow the FAA to supply the aircraft for maritime patrol. The RAF were supposed to do this, they used much of the money to build useless bombers instead. Patrol planes in 1939 remove most of the need to use fleet carriers for cover, so no loss of the Courageous.
Have the FAA order some decent aircraft. Either pay Bristol to build a 1200hp radial (not hard, it was proposed) to power them. The RAF never liked the Battle, but a version with a shorter wingspan (same area) would make a very good torpedo/bomb/recon plane. It was also stresses for an 80 degree dive, there's your dive bomber sorted. There were planes being developed that would make a good naval fighter (with some mods), but with no urgency as, again, RAF.
The FAA would have a lot more interest in heavier armament, they had a limited time to shoot a big plane down (interception of things like the Condor was a big need), pushing the 20mm cannon along.
If the FAA controls Coastal Commend, someone might actually ask to test the bombs, and do exercises against subs (something the RAF seemed to think was beneath them)
The armoured carrier (no, it wasn't an armoured deck!) was actually a marginal decision. If better fighters were coming, the equation swings more towards more fighters to stop a raid rather than trying to survive it, so keep on building Ark Royal derivatived. Without a new treaty, they'd probably come out at 25,000t or better rather than try and cram a design into a lesser displacement. That allows you 60-70 planes and decent armour protection (even ignoring the deck armour, the Illustrious class was TOUGH)
Light/trade protection had been a RN interest for decades. You do have some slips available (they are roughly cruiser-length, but there are limitations of equipment for cruisers, so lay a few down instead)
Build more destroyers. 3 flotillas a year is doable (just), either Tribal or J-class. You need to replace a lot of old, small destroyers, so push this as 'just replacements' and 'supporting the unemployed shipbuilders'. With more destroyers, you can convert the V/W's into escorts a lot sooner.
A/S ships. There is a problem building a lot of these early, manning them. They were expected to be build quickly, so build say 6 a year for a couple of years to experimental designs. Look for simplicity and speed of build, the use of diesels (for the long range versions). Then its a lot easier to start spamming them out.
A/S weapons. The RN had experimented with forward thrown deth charges, but it failed due to the misconception of using a single long range one that needed an accuracy that wasny doable at the time. have someone look at options (yes, this needs some money), and something like hedgehog is easy for 1939, and pretty easy to refit. The star-shell is known to be rubbish, why not develop Snowflake early? It's not rocekt science...well, actuall it is, but...!
The RN WAS looking at convoys. Thats why they started a few days before the war. What they didnt do was develop an escort group to go around the convoy. Again, takes some money, but with 4 years you can make those escorts a lot better, and avoid some of the early mistakes.
The RAF needs to be taken out behind the bike sheds and beaten until they show sense on bombers. As in, at least given them a decent bomb sight and navigation aids. And prioritise the fighters for the Merins.
Have the ramp-head Merlin die an early and unlamented death. That gives you 12-18 months lead on the later marks of the engine, more hp to help your planes. Instead of trying to develop loads of engines, show some sence. The Merlin and Griffon are all RR need to develop. Bristol can quite easily build a 1200hp radial, and the Hercules issues should have been sorted WAY earlier.
The Army suffered a lot from the decision that it wouldn't be sen to Europe. Admit it will be, that gives you 4 years to equip it. Some weapons development was leisurely, like the 25pdr and the Bren, speed this up. Design a decent tank for Europe, something around 18-20 tons, and insist it doesnt have to fire on the move. Something like a bit heavier, better Valentine would be great for 1939.
Go for a core of 6 Lorried and an Armoured Division as the core, then think on the territorials. Some short term service (again to reduce unemployment), say a 2-3 year term, would give a pool of young infantry to help build up the older men serving in the territorial units. If your going to be in Europe, some decent armoured cars would be needed.
Give the Army back the close support squadrons the Experimental armour used to train with. A 20mm armed Hurricane would be very suitable as a start. As would some 20mm guns to defend with.
None of these changes really requires magic (well, making the RAF see sence might)