I want to explore the aspect of full caliber antitank ammo again as I've learnt a lot more (including from british archives specifically) since this last post.
A ballistic cap certainly is worth using already in the mid 30s because even beyond the reduced drag, it also allows the nose of the core itself to be made slightly blunter (in line with interwar/WW2 APBC that is) and thus more effective against thick armor since unlike AP the steel core doesn't need to be made pointier to improve aerodynamics. As said before it was used by many other countries at the time, but Britain took until late 1942 or early 43 to field it for the 6 pounder, and 1944 for the 17 pounder (and certainly late for the 2 pdr as well). So that's a very low hanging fruit to improve performance even without wartime experience of thick armor.
As pointed out by someone else, the fielding of armor piercing caps for the 2pdr in wartime at least appears to have been constrained by limited facilities to develop and field it in time, although this is not confirmed. 6 pdr also went much of the war without an AP cap, and only 17 pdr got AP and APC ordered at the same time in 1941 (plain AP being a backup in case of cap shortages like for US ammo). To some degree the need for it was less crucial because high hardness and/or caliber thick plates and sloped plates were relatively uncommon so shitty underoptimized plain AP wasn't facing particularly difficult threats.
That said, there are still many arguments in favor of fielding capped ammo even before the war (when you can safely develop and produce it). Cemented and face hardened plates still existed even if mostly in thin plates designed against bullets, and the UK heavily considered thicker such plates for their tanks. It wasn't inconceivable that caliber-thick face hardened plates would be eventually fielded. Secondly, the British were already specifying and fielded thick plates in the mid/late 30s on the two Matildas, which badly overmatched the 2pdr and would promote core breakup. The high velocity of 2pdr ammo also already promotes shatter. In fact the Germans would field APCBC from day one based on naval practice, even on high caliber and low velocity ammo that were less likely to shatter. Now said caps were not optimal for the conditions of antitank guns as they were still only designed for 20° angle of attack and low velocity which is normal for naval ammo, but they still were useful and such APCBC was better than plain AP or APBC.
2pdr ammo at least also happens to have a relatively mediocre steel compared to ammo in other countries, so that it could break up to increase post pen effects. However this had the detrimental effect of limiting penetration capability, and frankly the increase in post pen effect against thin plates was not sufficient to justify that limitation. Note that this was part of why the 2pdr wasn't as superior to Soviet 45mm APHE during testing as it should have been.
Finally, as far as improving the 1937-42 situation goes, avoiding the atrocious 2 pounder APHE would be good too. Some people know that some stocks remained and were used in 1940 to disappointing effect. Now I understand why:
This is one of the worst if not the worst antitank APHE ever conceived because the filler cavity is absolutely gigantic and will heavily promote shell break-up even against bad armor. It would be actually closer to SAP, except SAP normally features an AP cap to reduce odds of core breakup. Regardless, the British should eiher skip this and go straight to solid shot like the French did in the interwar, or at the very least mitigate the limitations of APHE by having a proper cap and greatly reducing the size of the cavity, more in line with German practice.
So TL
R: ideally you want APCBC from day one with a better steel for 2pdr. 6pdr and 17pdr were less reliant on such things, but would still benefit from APCBC already being entrenched in British practice. However, optimization against sloped armor and mass use of tungsten or APCBCDS are not very plausible for the 1937-42 period, as British postwar archives note that sloped armor wasn't really democratized until 1943, too late to adapt the ammunition. Tungsten and sabot technologies are similarly a bit late to introduce so early, save for squeezebore guns with tunsgten cores. The French looked at those as early as 1936 (Larsen-Gerlich types) and hoped to field such guns in 1941.
Some people may ask why the 2 pdr would need so much love when the British faced thin armor up to 1941. The main answers I would give are that acting early would restrict the benefit of German face-hardened 50mm plates and add-on armor faced in North Africa. Moreover it still made sense even in the late 1930s because the British had heavily armored Matildas specifically to counter the 2 pdr, and the Germans certainly could do the same even if they didn't OTL (ignoring the French who already loved heavy armor). Pending the arrival of a replacement gun there was reason enough to get the most out of the 2pdr against overmatching plates.
One last remark on the 2 pdr before I go on another subject. I noticed that it used straight-walled cases, which combined with the small caliber makes me believe it was primarily optimized for towed AT gun use rather than tank use. It's not necessarily a bad idea per se and we know the doctrine didn't require good multipurpose tank guns anyway, just a remark. I'd point out the American 37mm gun used short, necked cases which is better for handling in the small volume of tanks, and the French did the same for their tank guns. Straight walled cases are ideal for towed guns because they optimize packaging to carry a lot of ammo on truck beds, but considering many countries accepted optimizing case shape for tank use, maybe 2pdr would have been even easier to use with necked cases. British 2pdr AFV turrets weren't exactly roomy.
Note that all of my comments on ammo design also hold for a potential 47mm Vickers tank gun. Regardless of ammo design, the appearance of thick plates on the Matildas also warranted the deployment of guns of greater caliber than 40mm to avoid being badly overmatched. This can justify giving more support to the 6pdr's development, and more specifically this should have warranted greater interest from the Royal Armoured Corps (the RA/infantry already wanting that gun anyway).
On the topic of better tanks, I would also make a new observation from the knowledge I gained since we last discussed this: unlike other WW2 tank-building countries, the British struggled to push the enveloppe in the period up to 1941/42. France, Germany and the USSR all made a big effort towards cast and welded armor during the interwar with riveted/bolted armor being basically inexistent by the mid 30s save for the remaining thinly armored designs the French produced in low quantities (AMC/AMR/AMD series). The UK is barely testing welded armor in the period and uses castings for only a few tank types, or even just turrets.
These "Big Three" also progress heavily in the field of automotives (dedicated high power/power dense tank engines, diesels, advanced transmissions and steering systems).
Meanwhile the British are doing well with transmissions, but almost nothing is done for engines with the Meadows DAV/MAT being too small for use in tanks beyond 20 tons, the Meteor literally only exists because some guy at RR was bored, high power diesels from Ricardo were never picked on, and the whole rest is just twin bus engines or the odd outdated aircraft engine Nuffield bothered to buy (and the Liberty was still an improvement over what the UK had!). The dearth of tank engine projects contrasts with other countries and with the sheer mountain of British aircraft engine projects.
Finally, the French go as far as planning optical rangefinders for 1941/42 and work a lot on gun stabilization, the Soviets test stabs in the 30s and the Americans ended up fielding one within a year or two of being in the war, yet the British do little even though they loved firing-on-the-move capability enough to deliberately design gun mounts to do that.
Basically, my argument is that British tank design probably lagged behind that of other countries in part because little effort was made in this field comparatively. Things improved after 1942 but the damage was done.
