I just checked in at UA LHR, and am writing this from the London Lounge. Both the UA premier/business check-in line and the fast track security at T3 were significantly longer than the economy/regular security lines. While it may be a temporary thing, it might be something to remember to check if you get the opportunity.

Also, the British government has instituted a new passport check right after security that creates an additional line.

In short, even if your flight is leaving on time, give yourself an additional 20-30 minutes for process if at all possible.

Also, it's been a while since I have departed from T3 and after you go through all this process you get dumped into a total maze of shops with absolutely no signage about where to go to get to a gate. Presumably, BAA likes the idea of having us forced to wander aimlessly through stalls for a while.