It's a long story involving delays and people sticking to original (not the revised) timings. FWIW I was at the gate before boarding had even started..I aso know that in my case seats and payload were available, but it was a "no", they wouldn't re-open the flight and they wouldn't accept me as an LMC to the loadsheet..
Bit frustrating as at outstations when being on the other side of the flight deck door I have regularly signed for LMCs..that's a part of modern LHR and some airlines for you....process driven.