Flight Departure Times
 

Could be for a host of different reasons, but always work on the scheduled time for getting to the gate.

The departure time showing in FR24 is the actual takeoff time and not the pushback time which is what the scheduled departure time is. At some very busy airports an hour of taxi time isn't unheard of.

Just looking at the last 3 days, pushbacks (against a schedule of 16:05) were
24th : 16:26, takeoff 17:02
25th : 16:43, takeoff 17:07
26th : 16:26, takeoff 16:56

which confirms the OPs observation that this flight consistently pushes late.

To summarise the OPs screenshots for easier reading :

Expert Flyer shows an average delay of 30 minutes over 2 months on this particular flight. Not sure if that's 100% true but I would think it's fairly accurate.

I'm seeing similar performance on BA11 (LHR-SIN) which I am due on next week. Hasn't been on-time for a few weeks.
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https://cimg7.ibsrv.net/gimg/www.fly...296fd8639e.png
I guess this is late departure from LHR coupled with longer flight times avoiding Pakistan (and now Iran?).
Oddly FR24 briefly showed a B744 scheduled on Monday which worried me a bit, until about an hour before departure when it reverted to A388. That part will be filed under the "FR24 isn't always often accurate with airframe allocation" I presume?
Could do with the timing improving though, don't want to miss evening cocktails and canapés at the Intercontinental!

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.

Unless you are specifically advised on the day of travel that you may remain at the lounge or, more precisely, that you may delay your gate time, you should plan on being at the gate prior to the gate deadline. Playing statistical games is a bad idea. That flight could operate an hour late on each of the last 89 days and on your specific date, operate on schedule.

It also is not worth timing around a 15-20 minute hold on the push while the taxi time is irrelevant to the passenger and is all included -- not added on --- to the scheduled time for the flight.

Since January:

Average variance between scheduled and actual (pushback) departure time is 18 minutes. Range is between -9 and 189 minutes.

Average taxi time (pushback to airborne) is 27 minutes. Range is between 16 and 55 minutes.