QWERTY9 is correct with the new Gatwick taxi-times. 20mins for 26L and now 25 mins for 08R in the summer. I can't remember what was decided for the winter.
Rather than due to a narrow taxiway, it was suggested and agreed by ATC that when busy, especially in the summer, it was sometimes difficult to get a/c airborne before the end of the slot tolerance during 08R ops due to the length of taxi.
An extra 5 mins taxi time was therefore added, and seems to be working, with fewer slots missed.
It is also worth pointing out that even if a pilot calls ready for start at CTOT minus published taxi time, in busy traffic ATC will sometimes deny start due to congestion and request a DLA message to be sent.
Some of the more 'helpful' controllers will let pilots know when they request clearance that it is busy and give them a time they may need to be ready by to make good the slot.