I have made a custard based drink once, it wasn't very good.
(I since learn't that it was because I put fruit in it it never thickened, like pineapple in jelly)
I tend to make my own dehydrated meals on the cheap but it will be a pot noodle emptied out of the plastic tub, smashed with a rolling pin and transferred to a plastic bag. Or those 9p Tesco noodles. (nothing but the best)
Vesta meals worked okay but took longer to thicken (poundland, £1 each, funny enough)
Oat-so-simples with powdered milk work well
Those one shot coffee sachets work with milk and sugar all in (I'm sure they taste like cack under normal day-today but cold and wet on a hill they are nectar)
Off course you'll need some buttered Bara Brith also

(a cake with an in built cup of tea)