The flight could arrive an hour early or three hours late. It could be cancelled. You might have a medical diversion to Shannon. Maybe the door won't open when you get to LHR. It could be that you're given a bus gate. There is no way to know for certain. If your flight arrives bang on time and you are the first off the plane, and there are no other people waiting at the e-readers, you can be landside in 12 mins, walk another seven or so to the HEX, wait on average eight minutes, and then ca 17 mins into Paddington. So, best case, 45 mins. Worst case, never. I don't have the data to plot all likely outcomes.