Surely that would be enough to make Britain firmly Democratic?
In terms of registration yes, although not usually in Presidential elections as far as actual votes go. My guess would be that Britain would have voted for Clinton in '92 and '96 (Perot would have done fairly well in '92), Gore in 2000 and Kerry last year. Bush sr. would have won fairly narrowly in 1988.
Mind you Britain would probably have been split into states rather than admitted as a whole country; Wales, NI and Scotland would all be seperate states and England would probably be divided into at least two states, maybe three.