AD 2-EGLL-1-13
Minimum Runway Occupancy Time...........includes......
"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."
Yes, separation standards are minima and may be increased when required. But there is no excuse for not notifying the inability to comply prior to accepting line-up clearance.
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. So asking for extra time once on the runway should no longer be happening. No wonder that point 5 gets irritated.