It is entirely sensible to reduce troop numbers. Nobody is going to invade UK, and expeditionary forces are prohibitively expensive for a country that's as deep in shit as UK.The strategic plan for Britain in NATO is to be a giant unsinkable aircraft carrier for the US. The nukes along British shorelines, increased number of recruits and manual conscription plans are in-case America doesn't come to Britain's aid. And with the changing of US presidency the aim of the US is 50 / 50 between a Biden supporting NATO government or a Trump America First government. The waters have never been so rocky and the foundations are showing cracks in the infrastructure so to say.
What UK should focus on, if they are concerned about invasion, is recruitment infrastructure. Look at Russia's mobilization in 2022. It had huge issues with recruiting men, to the point where highly valuable personnel ended up in infantry. UK should (and it can) setup the infrastructure to identify potential recruits, to have the physical infrastructure to process them, train them, and arm them.
Trying to beef up the standing army is a total waste of time. Adopting a 3-6 month civil service program makes sense, just to be able to keep this infrastructure active and not in disrepair, that way you can scale it up in war-time.