The one I've seen in PnS and Marketplace is a liquid in a 1 litre carton (like a milk carton), but isn't usually with the milk. Nearer to the yoghurt, cheese, UHT cream section. It's St Paul's brand in a blue carton, and is often next to a similar-sized yellow carton of St Paul's custard.
Like milk, it doesn't stay fresh for long, so maybe the powdered version is better value if you're just making small quantities of food. I've often just used a few spoonfuls and then had to throw it away a few days later.