I'm no more proud of being British than I am of being female, straight, 5'6" or a Scorpio. I didn't achieve any of those things by my own efforts, it's just an accident of birth. I'm very proud of some things that Britain has achieved, I'm ashamed and disgusted by other things Britain has done, before my lifetime and during. But when it comes to international competition, why wouldn't I support my home country's competitors?
I don't understand this need to be so negative about supporting Britain. I bet other countries would laugh themselves sick at the angst some people seem to have about supporting our own competitors.