Not sure if this should go in travel or in here, sorry if it's in the wrong place.

I'm planning a trip to London for science and history to go to the museums in July. As we live a long way from London it's going to take us about 3hrs to get there then we're getting the tube (route 1) to the science museum. We'll have about 6 hours in London altogether I think.

Obviously we aren't going to spend all day in there so what sights should we see and are any of the attractions free? What are the best (and cheapest) places to eat?