I'm in Windsor and have flown plenty of sites in the M4/M40 corridors and further afield:
Here's a starting point:
Google Maps Slopehunter
Ivinghoe (Dunstable) - West is best, can be a little crowded. bit of a walk
Whittenham clumps (Didcot) - bit small for my kit
Coombe Hill (Princes Risborough) - flat walk, can be a bit lumpy due to trees on the hill. (not been for a couple of years)
Uffington White Horse - Not flown personally, but have walked it and chatted to a few flyers on the hill - seems nice.
Beacon Hill (Newbury / A34) - BIG walk, but good slope once on the top
White hill / Watership Down (Kingsclere) - Nice, flat walk
Roundway (Devizes) - nice, bit of a walk, bumpy lane, not great for "low" cars!
Whitesheet (Mere/A303) - Nice, flat walk, friendly bunch often found on these forums!
Turville Hill (between Marlow / Benson) - A few spaces top of the hill, not a big hill, but can work with the right wind / model
Beyond that, a full on day trip is needed.... Rhossili and Bwlch can be done in a day from the Thames valley. South coast: Brighton to Newhaven etc is worth a day trip for the right wind (N. I guess most of the S options would work too). Colley Hill (Reigate / South of M25) is good in a straight S wind. Nice flat walk and decent parking too!
Cheers,
M