The only place I can see a major new park opening these days in Britain is in that destitute wasteland for theme parks - Scotland. It seems staggering that there isn't even a half decent theme park somewhere between Edinburgh and Glasgow, two of the biggest cities in the country. This is a complete guess, but maybe most of the time it's too cold for a full day outside? If so, why not a Toverland style hybrid?
Other than that, the north east sorta lacks a decent park too. I could only see investors going for Lightwater if the building restrictions weren't so crap.
The weather in central Scotland is pretty similar to that in central England most of the time. There is no reason why the Scottish weather should stop a theme park from being built! A decent theme park between Edinburgh and Glasgow would be much appreciated my me since I live between the two cities! M&Ds is a terrible park but it could easily be expanded if the money was there. Its placed in the middle of a country park with plenty of room for development but at the moment its squashed at the back of the park.