Agree with previous
If given "expedite climb until passing FL150" traffic is generally at FL140 or below.
Have been caught out before when I gave the expedite level as the same level as the confliction, and the aircraft, Airbus A321, reduced climb to 200fpm after the expedite and nearly caught me out.
Yes I know UK AIP states minumum climb in UK is 500fpm, most pilots don't notify ATC if unable however.
Cheers