The British are probably the biggest theme park fans in Europe. It's in our history from the Victorian seaside parks of yesteryear. Or to put it another way, I was at Alton towers on a cold windy summers day and the queue for the River Rapids ride was 2 hrs long. When I was in Spain we went to Terra Mitca on a nice warm summers day and it was a 10 minute queue for their Rapids ride which is just as good as the Alton Towers version. So the weather does not really come into it as far as the British are concerned.
If it was made to the same quality and themes as Universal Studios Florida it would be packed out no problem and that would just be with the British fans.