With 7 hours to spare, even with lounge access (well except the CCR) I would venture into London. The Heathrow Express train is conveniently located in the terminal building and takes 20 minutes to Paddington. From there you can walk to Hyde Park/Kensington Gardens, Oxford Street, Mayfair, Notting Hill etc. Lots of things to see and plenty of places to grab a bite. Starbucks and Costa Coffe have free Wifi.