I'm thinking 1933 or thereabouts for a starting point.
The QF 2 pounder has a historic great reputation as a "hole puncher", but for various doctrinal reasons an HE round was not put into use. How about the British license build the
Belgian 47mm model 1931 AT gun? It had very good anti-armor performance, plus it had a useful HE round. It was compact enough to be mobile and concealable. It was ready to go in the early 30's, so no potential for a tangled and disruptive developmental cycle (not that 2 pounder had any issue there, but the ordnance folks wouldn't know that upfront). Or, transplant the gun onto the analog of the OTL 2 pounders carriage, if you must.
I find the mv of the AP shot Belgian suspect - IMO too high for such a lightweight carriage. Barrel length is short, too. Look at the case and barrel lengths of the Belgian and of the "comparable" Czech gun - 20 versus 40 cm and L33 and L43 respectively. The Czech gun was light because of its primitive horse-drawn carriage.
So, something about the Belgium gun does not feel right. But I'm no expert ...
The 2pdr was too complicted and heavy. IMO the 2 pounder could be kept as OTL but with two small changes, best if
both:
- bored out to 47mm - and renamed the three pounder
- the HE shell follows the pattern of the Soviet 45mm, thus with more filling.
Simplifying the carriage would be nice too ...
Also, how about a updated pack gun in the 2.5" to 3" range? God knows the British had plenty of jungle and rough and rocky colonial country where such a weapon would have been useful. The Commonwealth troops in Malaya, Burma, New Guinea, Greece, Norway, and even East Africa would have appreciated a weapon like that.
The UK had the 3.7" pack howitzer - simply make more of them. Already in production.
I know, 100kg heavier than a 75mm weapon. But does 630 vs 730kg a difference make? When in "pack mode" broken down into parts anyway ...
As to other artillery - instead of the 3,45" 25-pounder either develop a new 32 pounder in 3.7" calibre, or stick to the 4,5" howitzer. Or develop a new gun in
that calibre round. We are taking vehicle drawn and rarely manhandled ordnance here, so whatever you go with will weight 1,5+ tons. IMO a 2 ton 4,5" howitzer is acceptable.
Oerlikon - buy it in 1935 and develop belt feed ASAP.
Pom-pom:
- go with the HV version ASAP
- simplify loading arrangements
- maybe stop with quads?
Although naval artillery is a separate can of worms