Well, that was a mess. Doing a turnaround from LHR T3 (for which there were check-in issues I will post separately) and the security queue started at the back of the terminal. Got to fast track split after around 45 minutes and through in an hour. People generally good natured (helped by staff calling out people from the line with closing flights) but such a shambles. I did enjoy the American woman in the queue directing queue jumpers to the back.
Does seem an ideal strategy would be to chill somewhere, join queue at 45-60 minutes before departure and wait for flight to be called!