Who would win Great Britain or the USA?

No one, we would destroy each other. For those that say we haven't enough nukes to totally destroy the USA, that's a bit of an insane comment to make. You don't have to blow up every bit of land to destroy a country, the fall out would ensure that what wasn't blown up was destroyed anyway. And let's not forget chemical weapons, I'm sure both countries have some of those somewhere. The likelihood is that whatever wasn't destroyed would be contaminated so badly that it would be uninhabitable, the water supply, crops, just about everything. If you've no water and no food then you die anyway.