Just to round this off.
Boarding starts: 14:58
"Final call": 15:15 (so they must have got everyone on pretty sharpish)
Boarding end: 15:26
Gates close 15:29 (exactly what I like to see)
Captain requests startup: 15:33
Startup OK: 15:41
Pushback: 15:41
Arrive 27R: 15:59 (not much of queue today)
Takeoff 27R: 16:04
Which counts as a good sequence of events for LHR.