My last two departures from Heathrow average at over 60 minutes. The queue for a 9R departure was on both occasions right back to the central area. In fact getting out in less than 30 minutes seems a novelty at Heathrow nowadays.
I'm not counting the one last year when we taxied in a BA 747, then on the way to the departure runway an unaccompanied transfer bag was identified on board. So to a remote stand where we sat, with a "suspect" bag under our feet, and waited 45 minutes for BA ground ops to put their tea down and organise a container lift to turn up and get the security risk bag out. Take security seriously ? Huh. Total taxi time 90 minutes on that occasion.
London City stands 8/9 to runway 9 can be less than 2 minutes !