custard powder is more commonly available in the UK. It is basically a cooked pudding powder. Whatever brand you have available that you add milk to and then cook on the stovetop would be a close enough substitute - although probably more intense in flavour (vanilla) than true custard powder and thicker. The thickness you can do something about by adding slightly more milk.
My wife is from Canada, we love Nanaimo Bars - but they're reserved for special ocassion - maybe like Valentine day, ....hmmmmmm
Use vanilla custard powder, cook it slowly and be conscience of the final consistency.
My wife has fooled her mom with this substitute.