This is not a bad approach. Seven hour layover is long enough but you will either spend a fair amount of time on the tube or pay for more expensive modes to get you into London. Do you need to leave anything at left luggage. That just adds to cost and time on your return to the airport if you do not arrive and depart from the same terminal.

One suggestion if you go earlier. London Walks tours.. they start at various times of days, at tube stops, run about an hour and half, dont need to be booked in advance, and allow you to stretch your legs and learn something new about the city and arent very expensive. The guides are either actor or scholar types.. weve done east end, hampstead heath, war memorials, warwick (little venice.. a really hidden gem).. they have a website and many choices to select from.