Cough,
dependant upon the departure aircraft, we allow about 45 seconds from take off clearance to rotation (and earlier, by wheels up i meant weight off). Often, in "normal" wind conditions this coincides with the point at which the preceding departure is about 2.25nm from the upwind end of the runways as marked on our ATM (radar screen) - this gives an alternate
guide
Ultimately, pilots can take more, but very rarely is it requested. TBF, the majority of EGLL departures are great and keen to help out. But sods law dictates that the one that really needs to get a shift on is the ditherer! In any case, if I require a prompt departure, I normally check and prewarn pilots. If their response is anything less than a resounding
, then it's not going to happen. And remember, we may want you to expedite in order to depart another aircraft after you, before the next inbound lands. Frustrating when for want of a few seconds, a gap is "wasted"