LHR clearance question
"Between 0630-1400LT and between 1500-2200LT pilots of operators who have been briefed with regard to the correct phraseology may call for ATC clearance up to 15 min prior to being fully ready to pushback. All other operators must be fully ready before calling on frequency"
Is this briefing something that is only available to the local airlines or something that is available to anyone?