Not convinced by this at all, whilst Thorpe Park is considered to be in London its over an hour from Waterloo either in car or via public transport, which actually makes it pretty inaccessible for your average tourist, especially when compared with all of the other more traditional tourist attractions that are easily accessible by the tube or buses.
Tourists don't go to London for Themeparks, Thorpe would have to convince tourists to take a day out of their busy schedule (the average stay in London is 4 days) which would likely mean them sacrificing something they'd already planned to do. Grona, Liseberg and Tivoli do it well because the parks are in the city's, you can walk to them and the parks have a reputation built by years of satisfied customers and excellent Trip Advisor reviews. The parks have embedded themselves within the city's top tourist attractions and will continuously attract the customers.
Even Phantasialand struggle to capitalise on Colognes tourism, a quick look on Trip Advisor and the park is nowhere to be seen on the City's top attractions. That was only a 25 minute taxi journey from the city center. Long of the short, tourists love convenience (they're on holiday after all) and Thorpe just isnt convenient enough to justify a trip from central London.
This is excluding the fact its just not a very good themepark