I thought it was a general speed limit in class A below FL100, not just for the major airfields, not just for departures.
No I was wrong. Found the AIC; it's Yellow 201. Trial is for 90 days from 8 Apr 06. Daft thing is it says 'airfields outside CAS do not need to cancel speed restrictions before giving traffic to a TC sector'! But there's a speed limit of 250 in class G anyway which airfields are NOT allowed to cancel.