I've just had a look at the BA live flight departures which I just don't believe. Not least that a colleague has been delyed getting to Geneva this morning and she said it was because the BA flight left late - well, not if you believe BA. It's down as an on time departure. Don't trust any business that produces its own reliability stats!
Entirely explainable: In the UK an 'on time departure' is recorded if the aircraft pushes back on time.
It might have indeed pushed back on time, but had a slot time, and a remote hold might have been arranged, and so it held on the tarmac at LHR until the slot time, thus it took off late.
Still officially an 'on time departure'.