It really won't matter if you don't get off the aircraft at the first open door you come to because you're going to find her to collect her and disembark with her. I would be surprised if there were a single member of BA cabin crew anywhere in the company who'd object to you wandering forward to WT+ to find your mum when she's waiting in her seat for you to get there. In fact, my experience suggests that many of them would probably be keen to help to reunite you on board before you all exit the aircraft together.