I was just about to say the same thing: why would anyone want to hang around at the airport (however nice the lounge) for so many hours, unless you wanted to get horribly drunk!

When we had a 6 hour layover at Vancouver, we went through immigration (where the Canadian version of TSA was astonished we simply wanted to go into Vancouver for a few hours, but showing him our upcoming flight info, a few hours later, convinced him), hired a car we'd pre-booked and paid (this was so quick from Vancouver) and went into the city. We had about 3 hours to spare and got a lot done - quick stop at a coffee shop, swung by the city, drove around and stopped at a park, had a walk, got some food, another coffee on the way back and all done easily within 3 hours. To this day I'm amazed how much we managed.

Remember at Heathrow, you're only 15-20 minutes away from Paddington, where there's plenty of things to do and see. In fact, I once got the Heathrow Express and walked from Paddington to Chilern Street, had a look around, got a coffee at Monocle Cafe and headed back. Didn't take long at all and was a really decent day out rather than sitting in Heathrow for hours.