I have spent a little time on the Thames in my SIB, so...
Firstly - have you found John Kennett's other site: http://www.thames-tideway.co.uk/
And for that non-tidal part up towards Maidenhead, I assume that you're expecting the 8kph (~4kts) speed limit?
Start by thumping "River Thames" into Google (if you haven't already).
As for camping - you're going to be travelling through some of the UK's most prestigious real estate, so I'm not sure if you'll find camp sites very close to the river. B&B might be better?
Don't know about petrol - 25 litres kept my SIB going all week at 4kts!
Don't forget to take fenders - the lock walls are very slimey and green