On the shorter domestics like MAN and JER, I've tended to notice the initial approach/arrival announcement tends to come once the flight deck know if there's any approach holding required, and the 20 minute call will come based on that, and may be inside of 20, and be more like 15 mins. It's roughly 10 mins from leaving one of the four stacks at LHR to wheels down, so usually the 10 minute call comes at the point ATC gives the aircraft the heading off the beacon to position for the approach.