Dave (udxr226) has actually summed up my thoughts on what the UK should be pretty well.
A full-on militarily setting already exists in Poland/Germany, so I would look to the UK setting to be significantly different, not just yet more bandits, but with different weapons and different accents.
There isn't really a credible mechanism* for large military forces in the UK, other than the British military (and survivors of US airbases). Given that the UK has low firearms ownership, it stretches suspension of disbelief to have significant armed forces all over the country. So a 'full-on' military setting becomes a version of Poland/Germany, but against 4th rate opponents, and is going to be a bit 'meh!'
As I've alluded to in some earlier posts, I was hoping T2K;4e would not just be the 'military setting' of 1st ed, but would provide options for a setting that included rebuilding: not just 'base building' for somewhere for the PCs to operate out of, but 'helping to rebuild society' type stuff. Still hazards to overcome (bad guys with guns, the environment, political games amongst those in charge, diplomacy with break-away regions or 'free towns', etc, etc). (And of course, we have yet to see the base building rule, so maybe this is all coming)
So a UK setting like 'Survivors' as suggested, possible with elements of 'the last train' (also a UK TV mini series from about 20 years ago) would seem to work.
Clearly different from Poland/Germany, with distinctly different challenges to overcome.
*Ok, if you really insist on having someone invade the UK, make it the French. They at least would have the capability to do it.