TBS [time based separation] on approach will seek to improve runway utilisation in windy conditions; much work being done in UK and Eurocontrol on this. Somebody mentioned Heathrow, which is often reported to operate at near full capacity. It may be politically acceptable to say this, but in actual fact the real movement capacity will never be realised because of severe operating restrictions caused by dedicated landing/take-off runway protocols.
How do ATC cope with extensive holding delays?...they can use both runways for landing subject to certain conditions. Compare Gatwick single runway utilisation to Heathrow's two.
A Clearance to take-off or land is not an instruction, it merely conveys an ATCO's situational appraisal [or judgement] so that the pilot can proceed....but the pilot makes the ultimate decision based on a multitude of factors of which ATC clearance is one. Certain UK ATC units have permission to clear an aircraft to land with preceding traffic still on the runway....not to be confused with the "land after" procedure.