We try to target to get there maybe 35-40 minutes before departure and that usually hits toward the beginning of boarding. Sometimes they board later and we hang, but not often.
Also, we now answer our "security" questions at the gate (and not the little room in Flight Connections) so we need a little more time. Since no one does it there, it adds like a minute, but I allow more.
Cheers.
If If you're starting from LHR, you get the security questions asked at check in, if you're a premium passenger.