What is there in the Pacific that Britain needs to fight for?
Okay, Japan can extend down towards Fiji, attempting to isolate Australia. But surely the priority is to take Burma, Malaya and the Dutch East Indies, which is not only going to be much harder (impossible?) without Britain fully committed elsewhere, but also produces a war that more resembles the WW2 Med, with navies and land-based air supporting land campaigns, rather than the "carriers everywhere!" of the historical Pacific war.
Exactly!
The Pacific will be a diversion for both.
In OTL the Pacific provided the battlefield because it was the way USA chosed to take to get from USA to Japan.
For the British Empire Singapore sets the agenda as the only fully fledged base and the pre war British plans also envisaged a trade war cutting Japan off from supplies and not at least utilising the substantial pre-war British fleet of ocean going subs but gradually closing with actual fleet operations. I have never seen any indications of ambitions of invading Japan - "just" starving them.
With the IJN preference for complicated plans and diversion moves you could of course to a degree expect them to exactly divert their assets. But this scenario is not about which strait to take around the Philippine Islands but about which Ocean to deploy in - thousand miles apart.
IMHO the big question is WHEN Japan will strike, they really don't have the option of not doing anything.
The article I posted will also be read in Tokya in March 1939, and although it clearly is propaganda loaded, nobody will be in doubt the the British Empire is rearming in an unprecendented degree (bypassing that of pre WWI!) and soon the Japanese chances vs. the British Empire will be non-exististent.
Here from OTL we know how great an asset the IJN naval aviation was, but the Zero only entered service in 1941, in 1940 the Val was still carrying out qualification tests onboard Amagi and Kaga and the Kate had just entered service in small numbers. And the IJN will also be short of the two big carriers (Zuikaku and Shokaku?) which entered service in OTL 1941 (along with Musashi and Yamato). Even the G4M Betty of Force Z fame was only introduced in mid 1941.
As other have pointed out, in an ATL like this the FAA (and the Far East in general)is likely to recieve far more ressources, but even if staying fairly close to OTL the Swordfishes, Albacores and Fulmars and night fighting capacity of the FAA would appear adequate compared to IJN Claudes and biplane divebombers and torpedobombers.
I would think that going down the South China Sea to attack Singapore or FIC would be more than even the most foolhearted IJN admiral would dare, but if focussing on Hong Kong you could perhaps hope on not only taking Hong Kong but also on luring out the RN and inflict some significant losses on it - and then negotiate! I would not appear that much worse a plan than OTL PH - but that really doesn't say much about its chance of succes.
Going directly for the Dutch East Indies would of course be tempting because of the ressources there, but conducting a successful conquest of DEI and maintaining oil production with Singapore still operating on British hands is a very bad plan...
And then the question of course is - what will USA do? It was not least FDR who put the thump screws on Japan for her engagement in China, and I guess USA would at least declare war on Japan if she attacks the British. Probably not with the OTL massive engagement, but I suppose the PI would be defended with what it takes.
I really don't see any likely path out of this for the Japanese, but if any chance attack as soon as possible - late 1939/early 1940? The Admiral in charge will probably write in his diary though: "I fear we have awakened a sleeping Lion..."
