As C-W-S says, there's a bug floating around which we encountered recently. We resolved the problem by using Twitter [via DM] with the helpful Social Media team, who managed to allocate us seats in the J cabin.

Seat selection on LGW 777s is becoming a right PITA. However, the CSD on our last return said that all the 3-class 777s are being re-configured to the 40J version, so at least that will be one less configuration to deal with