I'd recommend parking at one end of a tube line, it's usually a whole load cheaper and easier than going anywhere near London.
We usually park in Morden and get the Northern Line everywhere, but I most certainly wouldn't recommend that because Morden is a sh*t hole.
I would also say you're better off doing the Dungeons than Tussauds. Dungeons has been dumbed down quite a lot to make it 'family friendly', so it shouldn't be too scary (minus the Sweeney Todd and Jack The Ripper bits).
You really don't need to go to Thorpe that much... Go for one of the Fright Nights, don't bother with it more than once. They get incredibly busy and they really aren't worth it. I'd say try out something else for scare mazes if you can afford it. Tulleys Farm is supposed to be good, and from South London area should take no longer than an hour and a half to get there (driving).
Anyway, enjoy your time down south!