Apart from the passenger airports in the UK such as LHR, which handle large amounts of belly freight, they also have a smattering of dedicated freighter services as well (Asian carriers spring to mind.)
Due, I believe, to space/slot constraints, many other "large" freighter services use less crowded UK airports and then truck to London if required. e.g. Stansted (BA World Cargo 747s), Manston, Manchester, Prestwick etc.
East Midlands, Luton and Stansted can be considered as the major UK hubs for nightfreight services operated by the big integrators - linking up with European hubs, European destinations and some USA hubs with their MD-11s, A300s, 767s, 757s, 737s, 146s etc etc.