Most custard powder is just cornstarch and vanilla extract with some natural food colouring, but be sure to check the ingredients for whichever brand you choose. Some contain milk powder so obviously avoid these ones.
Again, digestive biscuits are usually vegan but this varies from brand to brand. Lotus Biscoff are also famously plant-based.
For your topping, if you can’t find vegan spray cream, you can always chill a can of coconut milk overnight and scoop off the solidified coconut cream from the top. Add a little sugar and whip until fluffy.