Attention all Londoners!

Hi friends:

I need some quick advice. I will be in London Heathrow for a 6 hour layover on Friday, starting in mid-afternoon. I'll be arriving from the US, and leaving for Africa.

Will I have time to actually go to London or to see anything? I would assume I'll only have about 3 free hours when you take into account getting back to Heathrow for the international flight out, and any customs stuff I might have to do. Also I expect to have my luggage with me, but maybe I'll check my small suitcase...

Any comments you might have, if you know anything about London, would be appreciated. I have never been anywhere near there before!