It's specified in the AIP what the minimum rates of climb and descent are (500fpm as stated). Can't remember the exact reference but it's near the beginning of the Gen section.
Reference the use of 'Expedite':
The example NS gives is a brilliant illustration of how many ATCOs think 'Expedite' will solve all their separation problems. The difference between normal and expedited ROC in an A340-300 going to Tokyo might only be from 1200 FPM increased to 1500FPM (and they can only do it for a few thousand feet) and if you rely on that for separation you're in the poo. I prefer to ask what rate of climb they could manage until through a certain level and then specify that. Then you are operating to a known value rather than an unspecified 'Expedite'.
Of course you should not be basing separation PURELY upon ROC/ROD but should have a radar solution as well. The increasing of vertical movement can help in getting aircraft going where they want sooner.
