Re: "A UK citizen is a UK citizen irrespective of how long they've been out of the country. "
Oh how I wish you were right. But Parliament has already made UK citizenship (and hence EU citizenship as well) revocable at the whim of the civil service. Source? I was reading my UK citizenship papers (not quite there yet) a few weeks ago.
That's for someone becoming one. If your family has a long history of being British and you were born in Britain to British parents (non-naturalised) then I think there's fuck all chance of that being revoked. Those laws apply to people that come in, get citizenship, then commit crimes etc. IANAL.