"When a man is tired of London, he is tired of life"- Samuel Johnson
I'm with Sam- you can't really have too much London. So much going on, and constantly changing so with every trip, we start planning the return trip there on the flight home.
Agree with others though that there is so much more to the country than the big city, and it's good to spend at least 2-3 nights further out to see other interesting cities, places, and bits of the countryside the British love so much. The university cities are good options, as is a Salisbury-Stonehenge-Bath route. While you still have lots of fellow travelers, it's not the mad crush you get in London, and you can actually stretch your arms apart and take a few deep breaths without having to worry about bopping a Chinese tour group or something. And there's something to be said for staying in the small town where you walk to a local bakery or cafe in the morning before walking on to see the cathedral or museum instead of starting your day with an hour and a half train trip followed by a bus transfer, or trying to put your rental car into Escher's car park.