in the mix. Its basically instant pudding, just add milk, whisk and wait 5 mins to thicken. It comes in a variety of flavors here in the USA...butterscotch, chocolate, vanilla, pistachio, coconut, lemon to name a few. The ones here come in 4 or 8 serving size packages. The size that your recipe requires is a 2-serving package of instant pudding, so for us to make it here in the USA we would just need to use one 4-serving package of instant butterscotch pudding mix. Our pudding here is not as firm as a custard.

You can probably order the mix online through some British export grocer. But, any instant pudding mix should do the trick.
Hope this helps.