You can never tell with LHR. A few years ago, I had to clear immigration at T1 and re-check in for a flight on a different itinerary. Total time from deplaning, immigration, bag check-in, and security was 20 minutes. Would I assume I can do that again? Not a chance.
My advice to the OP is leave a comfortable amount of time. OP appears to be 1K (*G) so enjoy the BMI lounge. It's pretty good.