Perhaps I'm misunderstanding but 1kg of whey protein on Amazon seems to cost around £20, which is roughly what I remember it costing back when I used to use it 7 or 8 years ago.

That's because the raw material is only ~10% of the retail price. According to EU data, commodity prices for sweet whey powder in the EU are about 1.6 € per kg.

https://agriculture.ec.europa.eu/document/download/413a6600-...