It's an interesting thought and I had considered it; I decided against it.

I just really can't see any European country isolating itself from the EU unless something catastrophic happens to the European Union, especially in a situation like Earth-2174 where the EU not only covers Europe, but extends beyond it. I don't see the Commonwealth trade make up for losing easy access to the mainland.

Besides, it's not like ALL of UK is against the EU, there are also proponents, and polls on the subject are likely highly influenced by the politics of the day. So for example right now, I'd imagine you'd get an 80%+ majority against joining the Euro; but this will look vastly different once Oil becomes less of a key ressource, and thus oil-backed currencies (esp the US$) lose in value against other currencies. If the Euro can survive the current unpleasantness - and I predict it will - then it's "weathered" - it'll gain more confidence than it's currently losing.

If the UK ever becomes a "dictatorship" of some sort - or otherwise run by elected fringe groups - then of course all bets are off, but again I can't really see that happen except by whim of the world builder.

Now, as for aggressors:

Current Commonwealth members that are close to China: Malaysia, Papua New Guinea (which we had join Indonesia), Australia, India, Pakistan, Sri Lanka. India and Pakistan are allies, and both are nuclear armed - they are unlikely to fear anything from China that the UK could protect them from.

Commonwealth members close to the US: A bunch of minor caribbean islands, and Canada. The US isn't going to pick a fight with Canada (not necessary), and even if - that's a situational problem, and not something that would fuse the Commonwealth together or even expand it.

Islamic Union expansion in Africa could be the most clear and present danger to the Commonwealth, but half of Nigeria already is in the IU, and again it's probably nothing that can't be dealt solely by expanding / renewing / strengthening the commonwealth.

So, long story short, I just don't see it happen :-)