"Pilots should ensure that they are able to commence the take-off roll immediately take-off clearance is issued."
"Pilots not able to comply with these requirements should notify ATC as soon as possible once transferred to Heathrow Tower Departures Frequency.
Believe me, all of us at LHR can quote the AIP until we're blue in the face, but most of us are more realistic. Hell, half the crews don't listen to the ATIS. We need to work in the real world, unfortunately.
ISTR that when I was an LL Tower Sup we had to log every instance when extra separation was requested. ATC Ops would then process the data, identify trends, and take appropriate action.
You'd be quite welcome to do my Sup duty on Friday and then tell me if you had enough time to log every instance as you describe. You're also quite welcome while you're in to inform our Ops Department that they should analyse said logs. In fact can I watch? Both they, and the Tower Sup, have far more pressing matters to attend to.