Link to UK AIC Vortex Wake
I agree with grasscutter, except that at the busier SE airport the 120 seconds is not used. Take off clearance is given when the previous departure reaches 2 nm so that the flight paths are probably about 95 seconds apart, it is then up to the pilot to decide whether he values his life sufficient to delay departure or not (I don't know if anybody would admit this though)
As for separations not based on Vortex wake, then as the airport grows, these are gradually reduced and procedures put in place to cope. Heathrow has various departures where 1 minute can but used between sucessive aircraft (no, not 60 seconds, but clearing the following aircraft once the first is showing daylight beneath the wheels), Stansted doesn't. But when conditions permit, then reduced separation in the vicinity of the aerodrome (visual separation) can be used instead.