Originally Posted by
Raffles
You've got enough time there to, for instance, go into Windsor and see Windsor Castle, or even get into London and do some - targetted - sightseeing. Science Museum, Natural History Museum (all the dino skeletons) are on the Piccadilly Line, South Kensington Station, which is the direct line to the airport. Tube would be 45 minutes each-way and the full 'terminal to museum front door' trip would only be an hour.
+1 Windsor castle. It is STILL on my list of things to do though...