Mixed operations!
At airports where you usually use one runway for departures and another one for landings, like EGLL, then the pilots may not be aware of that there could be landing aircraft on "their" runway but when you have landing and departing aircraft on the same runway, called mix operations, then everybody have to be more alert and ready for action. Normally, if you as a tower controller, wants to depart one aircraft before a landing aircraft, you make sure that the departing aircraft is ready for a rolling departure. This makes everybody alert and ready for action.
If one ATC unit has a "relaxed" approach to issuing landing clearance (12NM from touchdown) then that unit is in trouble and should immediately change their way of working. This is not a safe way of working.
One way of improving the working methods is to have a "no blame culture" which means that you report everything that happens and a lot of effort is put in to making the job safer for all involved. This is in force in Sweden and works well.