Make sure you have plenty of patience. The process is a bit of a disgrace, and the lines are generally long, slow, and completely understaffed. They even invented a nowhere-else-in-the-world "conformity" check (which is understaffed and has its own line) which achieves nothing more than make the connection even more stressful.

T5 it's one of my least favorites connecting places in Europe, and if it weren't for BA being the best AAdvantage airline to fly transatlantic to Europe in J-class and the quality of the BA lounges at T5, I would never voluntarily submit to that hassle.