I have done the trip into London many times on layovers, and 6 hrs is more than enough time. If you take the tube to Piccadilly and factor 45 minutes each way for the trip this leaves you 2 1/2 hrs in Central London and then 2 hrs back at Heathrow.

This leaves plenty of time for a nice walk, pub lunch and a few pints in one of the worlds greatest cities! I would do this any day over hanging out in any lounge.