Yes Positive Boarding is used at T3, but it's something silly like 25 minutes. You really would not make the flight in that situation. Only applies at landside entrance, not Flight Connections. Some airlines do publicise the timings, MH springs to mind. In the case of BA it would complicate the 45 minute publicity for the check-in deadline. It's no different from the unofficial version that exists at many airports all over the world where the person checking your boarding pass looks at their watch and says "no chance mate, go back to check-in and see what they can do for you".