Seven hour layover in Heathrow

You'd only have a couple hours in town. Decide if the effort is worth it. What about luggage? ATM's abound, use them.

You have time for a very short visit. Don't underestimate how long it takes to transit through Heathrow.

You don't have time to go to London, but visiting Windsor is doable. It borders the airport and is very easy to reach via taxi; about 7 miles away. It's home to Windsor Castle, which is open to the public, and there's excellent shopping and dining across the street.

http://wikitravel.org/en/Windsor_and_Eton

You can get to Paddington Station in London fairly quickly via Heathrow Express or Heathrow Connect. Luggage can be checked at the train station. Underground and buses (including the Hop-on-Hop-off buses) have stops in or near Paddington. Find an ATM at the airport. Don't count on more than about 2-3 hours in London, but that's enough time to do some touring and have a meal before returning to LHR.

We had a 9 hour layover in London and we checked out bags at the airport (not sure if all terminals have?), we spent the money (it was my first time in Europe) for the train into downtown and walked around for a few hours (it was good to walk around). It was worth it for us for the cost and the trains are super-efficient.

Unless your luggage is checked through, I agree with Michael that you don't really have time to justify going into London. It can take up to one hour to go through immigration. There is likely to be a line at any of the Heathrow left luggage offices. It will take a little under an hour to get to central London on the Tube. (Taking either the Heathrow Express or the Heathrow Connect doesn't save much time because you still have to get from Paddington to the areas with historic sights that you can walk by.) You need to be back at Heathrow two hours before your plane leaves. You'll probably encounter another line when you retrieve your luggage. So again, unless your luggage is checked through, you'll probably have no more than two hours in London.

Tim is completely right about the time, to be safe I would suggest going to Windsor. You can get to Windsor in 15 minutes by taxi. Tour the castle, have a pub meal, walk a round for awhile then get a taxi back to the airport. You need to be back at least 2 hours before your departure time.

Been to london many times (over30) the Windsor idea is the right one.

I'm going to agree with the suggestion of Windsor. I don't know why a bus was mentioned as a cab ride is short and easy.

For that short amount of time, there are just too many variables that can go wrong if you attempt to get into London. Trains break down, lines longer than normal, etc. And whenever you're on a tight schedule, that's when things go wrong.

Enjoy Windsor and save London for when you can really spend time there.