I think only certain gates at T5A can take a 767 - those along the 'front' of the terminal rather than at the 'ends' if that makes sense? And I don't know if these gates have the photo reference system or the ability to feed pax into the UK arrivals system rather than International.
Looking at this pic it seems only one of the North end gates could fit a 767 (the A320 and A321 in the picture are fitting their gates completely). Can the gate on bottom right (occupied by a A320) feed pax into UK Arrivals?