Quoted verbatim from the MATS Part 1 that Gonzo mentioned.
When aircraft are using the same runway, a landing aircraft may be permitted to touch down before a preceding landing aircraft which has landed is clear of the runway provided that:
a) the runway is long enough to allow safe separation between the two aircraft and there is no evidence to indicate that braking may be adversely affected;
b) it is during daylight hours;
c) the preceding landing aircraft is not required to backtrack in order to vacate the runway;
d) the controller is satisfied that the landing aircraft will be able to see the preceding aircraft which has landed, clearly and continuously, until it has vacated the runway;
e) the pilot of the following aircraft is warned. Responsibility for ensuring adequate separation rests with the pilot of the following aircraft.
Criteria for the "After the landing, cleared to land" in use at Gatwick and Heathrow. Taken from the AIP.
(a) When the runway-in-use is temporarily occupied by other traffic, landing clearance will be issued to an arriving aircraft provided that at the time the aircraft crosses the threshold of the runway-in-use the following separation distances will exist:
London Heathrow and London Gatwick
(i) Landing following landing - The preceding landing aircraft will be clear of the runway-in-use or will be at least 2500 m from the threshold of the runway-in-use.
(ii) Landing following departure - The departing aircraft will be airborne and at least 2000 m from the threshold of the runway-in-use, or if not airborne, will be at least 2500 m from the threshold of the runway-in-use.
SH