de facto
At airports like LHR and LGW you are required to be in position to roll by the time the previous aircraft lifts off. If you are still p***ing about lining up when you are cleared for take-off you have wasted very valuable runway capacity. At these and many other airports the cumulative wasted capacity has an enormous cost in congestion, wasted fuel while queueing, arrival/departure delay costs. The cost runs into millions for the airlines concerned and has a significant environmental impact too. If you think its okay to take your own sweet time unless ATC have told you to expedite you are not only selfish and wasteful but are also in breach of the UK AIP.