Originally Posted by jerry a. laska

You will need to factor in time to go thru immigration upon arrival and to get through security upon your return. That said you have time to go into London and see something. You could also head to Windsor.

If you can afford a mincab or car service do that, otherwise take the Piccadilly line in.

See these threads for ideas:
http://www.flyertalk.com/forum/u-k-i...over-kids.html
http://www.flyertalk.com/forum/u-k-i...ation-lhr.html
http://www.flyertalk.com/forum/u-k-i...r-layover.html
http://www.flyertalk.com/forum/u-k-i...yover-lhr.html
http://www.flyertalk.com/forum/u-k-i...few-hours.html