Because it's USA, 5C is unlilkely, 5B is where the USA security team is based. There is a good chance it will be 5B to 5B, if not 5A to 5C. If there are only short queues for the e-gates and it's moving, then use that + First Wing, avoiding the checked luggage queue, just go to security. That said, unlike MAN, LHR's problems are not really in the security area, they are in checked luggage / docs check plus ground handling. So pack your checked luggage accordingly since there is a chance of it being delayed a few days, and take a photo of the items laid out on the floor or bed before you pack them.