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.
while I agree the gate is often open beyond the T-20, conformance does not usually remain open. The gate team at least know that the people not boarded at T-20 are in the terminal so they can have an idea how much longer to keep the flight open