In London for a day, got a question

I'm going to Europe in May next year and im stopping in London at Heathrow for about 11 hours for the connecting flight. I was wondering if any of you brits have an idea how long it would take me to get from and to the heart of the city, maybe see a couple of places and get back in time for my next flight.In London for a day, got a question
Get a rail card, you'd be zipping in and out of different towns and area's using the underground.My advice, go and see the street performers cos its real funny as well sightseeing and shopping.
Theres info places everywhere and people can help too. Just dont go to a bar or club or you'd up staying there.Dont wander off too far otherwise youd get lost. London is huuuuge. I always use satnav.