Swap for Cake Margarine in the U.S.?
My understanding is that "cake margarine" is regular margarine which has been whipped to incorporate air. As long as you're using a stand mixer to cream the margarine and sugar (and do so at high speed), I suspect regular margarine should be fine, as that creaming step is intended to accomplish the same thing. If you chill the mixer bowl in the fridge, it'll help avoid overheating the margarine during creaming, giving you more leeway for a longer beating time.