1.Check out Dorney Lake, you can park up before the gates, unload the bikes and ride around it for an hour or two. Also, starting from Dorney you can cycle the tow path all the way to Windsor / Eton. Not ideal for a road bike but if it is dry enough it is OK.
2. The back roads around Twyford and Waltham St Lawrence are fairly quiet all year round, so From the B3018 go down Hungerford Lane, up Brook Lane, School Road, Sill Bridge Lane etc. You could head all the way to White Waltham Aerodrome and back, its a nice ride and its mostly a 30 or 40 max, I don't think there are any National Speed Limit sections / 60 - but I might be wrong.
Hope that starts you off at least. Its a really tricky one, I have a 10 year old Son who wants to ride with me.