The only time I've had this was when my MAN-LHR flight was delayed. As we were checking in our bags, the agent noticed we'd be very tight, so pro-actively asked if we'd like to move to the preceding flight.

If your MAN-LHR is on time, I doubt they'd move you. More often than not, you'd arrived into LHR early or at least on time, as the MAN-LHR flights have a 1:10 flight time, when you're in the air only around 35-40 minutes.