We'll arrive around 8 and take the airlink shuttle to the Busaras Bus station and rent lockers here for about 5E. The rest is pretty flexible but theres a double decker bus tour that loops the city; its the kind of thing that you can get on and off all day at like 20 stops. Its 15E, but if we book online we get 20%off. Once we've done the loop we'll get our stuff and take the bus back to the airport.
We ought to check out and leave the city by 2:30-3:00 so we can make it to Canterbury. The cathedral closes at 6. So, we can go to Westminster, take the tour, then bus/tube back, get our stuff and drive to one of the markets or even just around the city. We'll just wing it.