We used to have Merlin passes, so we know all the theme parks pretty well.
If you live anywhere near Hampshire, would recommend Paultons Park. It is outstanding. Simply amazing. The park is family run, not owned by shareholders or big CEO's interested only in making money, it is clean, rides are fantastic, Peppa Pig World is heaven for little ones....and best of all, queues are brilliant. Even on a Saturday, queue times are at worst 15 minutes.
Yet to see the school holidays, but they don't have a fast track system, so queue times aren't increased.
Having grown up in London, I can honestly say the likes of Legoland and Chessington, are really quite pathetic in comparison.
One good thing about Alton Towers, is they often let you go the next day for about £9 each. So if you do spend the hefty sum getting in one day, you can go back the 2nd day much cheaper.
Have fun whatever you decide! 