You seem to be missing a few points regarding the London/Kent project.
What is being proposed for the Kent project is something far bigger than just a theme park. LRCH are proposing to build a Entertainment Resort!
Once completed the resort would included; Cinemas, Theatres, Restaurants, Hotels, Exhibition Centres, Entertainment Streets, the biggest Waterpark in Europe, Monorails and of course a Theme park!
All of which would be twice the size of the London Olympic park! Just look at the difference in price for each project;
Paramount Park Spain - $500M
Paramount Park London - $3.2B!
Although the weather in the UK is not great, certainly the Southeast of England does enjoy some of the best weather and generally the highest temperatures of the year can be found in Kent.