The units are SI in the UK, but retailers appear be allowed to mix and match. This is particularly noticeable with milk in Tesco, at least last I looked - the price of the 1pt and 2pt milk cartons were priced in pence/100ml (~£0.09/100ml, I think), and the 2L+ ones were priced at £/L - ~£0.55/L ;)

I now buy the 2L cartons...