AOG, not much. Having asked this question in fact on another site some time back, got much the same response, even though others who perhaps were military, or ex military said that come a major war that you wouldn't have a choice. Forgetting one important part of this, that we have many people here in Britain of somali, asian, Afghani and so on, backgrounds, so how would they be expected to pick up arms, possibly against their countrymen and women. I am sure many would leave Britain before that happened. Add into the mix that though many from these nations like India, did fight and die for this country in both world wars, time has moved on, and we don't have an empire, nor have control over any of the dominions that added millions to the British fighting men and women.
An b/f who had been in the military, once told me next major conflict wouldn't be a land war, too expensive and too costly in lives, but as already mentioned a nuclear war, where no one will be the winner.