I used to work there when I lived in London before moving here. Are you interested in planes? There are lots of good watching in the airport. You used to be able to move from terminal to terminal as long as you didn't go to domestics you could cover lots of distance. They have chapels there if your a godly guy? Lots of shops it's just like being in a Mini mall but prices are hi. You can ride the connections train from terminal 1 and 3 to terminal 5 or catch the bus to terminal 4. Man I envie you having such a long stay with lots to do
have a good trip.