A part of this was due to Mr Churchill when he went back to prewar Gold Standard
A good POD for a healthier British economy for WW2 would be for Churchill to to keep fiat currency. It has always struck me as odd that Churchill was so unconventional in so many ways but when it came to matters of economics he was very orthodox.
If you really want to start w^^king the situation have the BOE do the 1920's equivalent of quantitative easing (printing funny money) to help pay down the British national debt. Sure inflation might be 1% or 2% higher (in a period of relatively low inflation) causing the economy to slow down a little. However it might just be enough to see off the default that hurt Britain's ability to raise loans during WW2 which lead to; first Cash and Carry, then Tizzard then Lend Lease. These collectively so damaged the UK's economy and comparative advantage that I've seen it argued the UK's economy only really started to recover in the 1980's.
Paradoxically if the UK had to find debt repayments right up to and through the war there would be less money available to pay for shiny military toys like trucks, tanks and planes especially pre war. However the long term effect of having US fuelled credit would more than make up for this during the war.