If you only buy milk in multiples of 500mL, you're buying a luxury brand or from a corner shop -- both situations use the slightly smaller (vs 568mL) size to make the price seem lower.
You can see at Tesco [1] that Yeo Valley, Arla, Cravendale, Finest, Tesco Filtered are all 1 or 2 litres. "Normal" milk is 2 or 4 pints.
(The EU used to have regulations around this sort of thing, but I think they've unfortunately been repealed for most products. It's why bread was only sold in multiples of 400g -- it meant the merchant couldn't discreetly reduce the size of some/all products.)
Asda only has it in 500ml and 1l containers. I couldn't be arsed walking to Tesco, and the nearest Morrisons or Sainsburys is a 20-minute drive away.
The dairy that delivers here does 500ml, 1l, and 2l, as does the one I bought milk from before I moved, and the one before I moved the time before that.
