I agree the decision was fine, because the freezing level was (presumably this was UK) well above the MSA so you had a way out which was to descend a bit. I do the same. What I meant was that the possibility of ice accumulation was to be expected. But it sounds like you had a turbo/turbine aircraft (2000ft/min sustained to 10k ft isn't bad!) which is something else (even if it was not "known ice" cert)