I think it's general knowledge that the UK military is a paper tiger, I think Charlie Stross said something about it being enough to defend one small village or something like that (he occasionally comments on this site so may correct me).

I think that damaging what little remains of its defences, which may exist mostly to keep the nukes safe so nobody tries anything, is still a really bad idea. Especially given that the US is increasingly unstable and seems like it may stop responding to calls from assistance from anyone else in NATO, and the UK isn't in the EU any more and therefore can't ask the entire EU for help either just the bits that are also in NATO. Theoretically the UK could also ask Canada for help, but right now it seems more likely that Canada will be asking all of NATO except for the USA for military aid to keep the USA out.

(What strange days, to write that without it being fiction…)