Ok, Three days in London Is plenty.
The places you have already listed are a good start TBH.
Also Try to get to a Football game at either Arsenal,Chelsea,Fulham, West ham,Charton Now that would be an experiance!!!.
Also Sept 3rd is Trax, an event at Silverstone,
http://www.traxshows.co.uk/ 1 hr north of London, where all the car clubs meet for a massive track day, come along and I'll take yer for a ride!!!.
Personally I would not bother with Stone henge. DULL.
I live about 2hrs from London, but Rarely go there, I dont think it is a true representation of England, Being so many Imigrants there, but thats another story.
You could always get on the train out of the Capital after a day, and head to
Cornwall, Stunning sceanery, check out Tintagle
http://www.tintagelweb.co.uk/
http://www.padstow.com/
To choose a few!!!
I'll look up things in London or around there for you.