Layover in Heathrow

You might be able to swing a quick trip to Windsor, which borders the airport. This was discussed in another thread a few days ago.

You would not have time to go to central London. You may have time to do a short trip to Windsor but your 5 hours will quickly reduce with the time taken to change terminals etc. If it were me I would just spend the time in Heathrow - as awful as the place is, there are plenty of shops. Heathrow or Windsor are the only real options I think.

It's a 15 minute taxi ride to Windsor. Public buses take about 30 minutes and they run every 30 minutes. With your limited time, I'd take a taxi.

I would not advise that. Do a bit of shopping in the terminal lots of shops and sit down have coffee read paper and it is time to go. I also had 6 hours on my trip to Prague through London and I might have been able to do it, but everything would have had to go right.

If you land at Heathrow on time, then I would agree with Tim. Head straight to the taxi line, take a 15 minute ride to Windsor, then you could either tour some of the castle, walk down to Windsor Great Park (which is only three blocks from the front of the castle) its a great place to take a picnic lunch, see the Long Walk which is the private road for the Royal Family to enter and exit the castle, you have a fantastic view of the Royal Residence from the park, could walk three blocks in the other direction and cross the bridge into Eton, or just sit on the bridge and enjoy the swans and boats on the Thames. Spend 2 1/2 hours doing what ever you chose to do, take the 15 minute ride back to the airport and you'll be there 2 hours before your flight. Or if you more comfortable spend only 2 hours in Windsor, getting back 2 1/2 hours before your flight. Have a great trip.