Originally Posted by
NickB
This is the official position but, IME, it is rare (on short-haul at any rate) for boarding to be completed at that point and the gate almost invariably closes later.
I agree, never once seen the gate close at T-20 at LHR (or LGW pre-Covid). Being mildly claustrophobic I will wait until I see the very last 5 or so pax in the line and only then join the queue. Usually this occurs at around T-10.
Invariably from my frequent experience, at T-20 there are still some 50+ pax in the queue from the lower boarding group numbers.