We used Travelsphere when we went at the 'Golden Time' - end of September to early October - 2 years ago. They were great! You have to keep going - no lazing around. You do see all the main sights though.
There is one in London its on the Ring Road east think it is near Kings X but dont know exact address meant to be good its before St Pancras Stn have driven past so many times but cant thikn of exact road name but defo near that bit