In my experience, 20 minutes taxiing and queuing at LHR is close to the minimum you see, it only gets a bit less early / late / Saturdays when things are quiet, and you're in a nippy A319 or something. They won't push back from the gate if the queues are too long, or if they do they'll park up somewhere off the airbridges for a bit.

40 minutes is my "going rate" for longhauls, over an hour is a bit tedious but not unheard of.

So at least some of the timings above are consistent with on time performance since this will be padded into the schedule. Thereafter jetstream is what it is.

In terms of the OP's question, boarding can and does close 20 minutes before advertised departure time. It's often a bit later, but particularly bus gates you can't count on it. But that is the time a passenger needs to regard as the final time, and of course you need to get to that gate.