The question of going to London during a layover has been discussed many times in the UK Forum and there are some excellent tips on what to do in that Forum. Depending upon the sites, it’s often quicker and easier on the Picadilly Line - eg to South Kensington for the Museums and decent food, Green Park for Buckingham Palace and the Royal stuff, or with a cross platform change to the District Line at Hammersmith to Westminster for Big Ben, the Abbey and all those goodies.