"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.
Of course I agree with every position the civil service takes on everything. Equally I am certain it would never do anything other than for the good of the nation and UK citizens. Those they consider to be citizens this week, anyway. Or next week. Or whenever.