I've done that Cape Town - London flight! I was on SAA and they had personal screens so I watched lots of movies which helped kill the time. Hopefully
Fobulous' plane has the same option.
I also had one day in London. You can hop on the tube from Heathrow and hit any part of London. I went on the
London Eye which is fairly central and is close to many other sights. You definitely want to avoid shopping there-man, is that city expensive!