You could try the Hinds Head, which is next door and also run by Heston Blumenthal. Does superior versions of traditional English cooking, and much more pub lunchy than the Fat Duck anyway.
Bray is a 5 minute taxi ride from Maidenhead train station (about £8 probably), which is 25-40 mins from Paddington. Taxis run from outside the station. 01628 770711 are reliable for your journey back.
If you fancy a walk, you can walk along the far side of the Thames from Maidenhead, past all the stupidly expensive houses (Parky, Rolf Harris etc) and Bray. Annoyingly there's no way to cross the river at Bray, so you'll have to go back to Maidenhead to start your walk, but it's not far. Take River Road, on the east side of the Thames, and walk south.
Why don't you do what every other cockney in the world seems to do on a Bank Holiday weekend and come to.........wait for it ..............BRIGHTON.. especialy the dozen or so in this office including the ones who had the audacity to ask 'as parking is so bad in BrighTON can I park by your house' no you fcuking cant play up your own end.