If you aren't achieving 500fpm, you must advise ATC.
'Expedite' means highest available rate of climb or descent. It has no numerical value in the UK.
Most ATCOs have a good appreciation of aircraft performance. Some don't. A long, long, time ago, I was on my way with a fore-runner of the present Mrs NorthernSky from Oxford to La Rochelle in a knackered old Seneca I (ex-OATS with the spare instrumentation to prove it!), which I had borrowed from a chum (my usual love-machine being on maintenance).
I was trying to coax the old girl up to FL110, and passing about 88, when ATC called: 'Golf thingy-whatnot, expedite until level'. I replied: 'I was just about to report that we're unable to maintain 500 feet per minute, Golf thingy-whatnot'. We ended up with a big turn and the traffic passed clear.
I guess said ATCO 'learnt about flying from that'.