The army has been hollowed out along with everything else:
https://news.sky.com/story/us-general-warns-british-army-no-...
EU has become less important militarily. Partly because UK left but mainly because Ukraine war has energized NATO. Sweden and Finland wanting to join NATO will get rid of potential conflict that only EU could respond to.
I cannot parse this. US is part of NATO, but so are countries (even if not all) in the EU. So NATO being revitalized can also revitalize EU, or the US, or both, in different degrees.
The EU army isn't very plausible post-Brexit because only the UK and France had decent militaries but it's technically still EU policy.
So basically, not much chance of this happening in the UK without substantial pain.
I'm serious, the pollical class clings desperately to these illusions and will not give them up easily if at all.
The conclusion of your first sentence would be the solution is in the cause - the end in the beginning. If the UK can resolve its identify crisis and understand its reality as a medium sized country in europe then it can begin to make rational decisions.
For instance, if it sublimated its defence to the EU or NATO, it would not need to have a first class army. If it re-integrated with europe (via the EEA, I don't think an EU re-entry will be possible on either end for a long time) then a battery factory in France still enables a UK car industry.
It would take a miracle to align the UK's reality with its self image, but aligning the self image with reality just requires a more honest media.