Success!
The delayed BA1338 to NCL last night, to the consternation of many, suddenly found itself departing from B45, with the instruction on some displays in T5A to "BOARD NOW."
After a reasonably relaxed stroll through the tunnel, I arrived at the gate to hear an anouncement for anyone who needed more time, and those in Group 0, to come forward. Just me in Group 0, so was let through before the crew, waited near the top of the stairs and was the first passenger to board.
(They then let all those who had gate checked a bag on board, before Group 1..)