If we repharse the question as to "latest point to GA without landing clearance" I believe it is not defined? (well, it is for us in LVPs). 20' might be an answer, but it depends on what is going to cause NATS to run to the press
Are they going to be upset if you briefly touch down on the GA? If so, then as much as height being important, it is the point at which you reduce thrust - since this determines the spool up time making a touchdown more likely.
I never learned a fixed altitude for that either. I only know that you have to ask on passing the OM if there is no clearance or statement on when to expect the clearance.
Touching down while in the GA might happen in a heavy when the GA ist initiated at the CAT II or even CAT I minimum.