Welcome to FT. You should be fine with a 90 minute connection. As is the nature of LHR, inbound delays do occur but even when you're at the sharp end and facing those progressive delays that eat into the best of your connection window, BA can get you from the aircraft steps to the Flight Connection Centre in under 7 minutes if needs be. I had the luxury of such a transfer last month in the back row of a BA liveried Ford Galaxy.