Comment by tialaramex

If you buy beer "loose" like at a bar it has to be sold in pints. Most people will have seen a "half" and anybody who likes stronger beers or goes to festivals where you taste different ones will know a "third" of a pint is also a legal amount of beer to sell. You would not want to try out a few different 8% stouts if they were sold only in whole pints, unless they're going to make it a multi-day event and provide somewhere for you to sleep it off.

Milk is also allowed to be sold in pints, traditionally glass bottle re-usable milk bottles were one pint.

It is also usual (but not legal) to sell a pint or a half of various soft drinks, in theory you should be sold these in some other way, I always say "large" or "small" but in practice ordinary people say "pint" and after all the staff will probably more or less fill a pint glass so, whatever.

Spirits (e.g. gin) are measured in either 25ml or 35ml shots. An establishment can choose either, post which one they picked and use that consistently. Why two seemingly unrelated sizes? Well, historically there were two different non-metric sizes permitted in law, and when the government legislated to make these SI units there were lobbyists demanding they allow this to continue despite the opportunity to rationalize.

As in the US, containers you purchase in a store are labelled, but here the labels must prominently show SI volume units and EU-style value metrics are required on shelf markings, so e.g. 10p per 100ml of Coke is a good price, maybe the Pepsi is on a deal for 9.5p per 100ml, the store's terrible own brand is 5p per 100ml. This EU strategy prevents people screwing with sizes to make you think you're getting a better deal, that cheaper bottle may look like a good idea but hey, it's 18p per 100ml, ah, it's slimmer in the middle which makes it actually much smaller than it looks.