Interesting, is that an EGLL-ism?
I suppose the reason one would say that WT cannot be reduced, as a EGLL controller, is that we rarely have any of those situations you refer to.
As I am sure you will agree that while allowing one pilot to conduct a visual approach is nice and easy as a APS controller, it does come with a certain degree of uncertainty with regards to the spacing of subsequent aircraft. With EGLL being more or less non-stop landings all day, the final spacing is crucial and cannot be left to the pilots to manage themselves - no offence
Similarly, do we very rarely, if ever, have small aircraft in the circuit which you would advise of the recommended spacing behind the preceding etc.
So practically, at EGLL, WT separation cannot be reduced.
Just my two cents
