In the UK, Imperial (284ml - i.e. 1/2pint), metric (250ml), and 'US customary' (237ml - i.e. 1/2 US pint) are all commonly sold. Less common is them actually telling you which they are.

In fact they mostly only say if they're US (though not stating that isn't a guarantee they're not!) because that's a selling point - people almost exclusively want them for following American recipes, they're rarely used here otherwise.

But for some reason that doesn't stop the others being sold.

Recipes also rarely state which. Or they'll say '250ml' but you suspect they're probably actually just rounding from US cups.

It annoys me out of pedantry and ambiguity, but with my other hat on, I think people care far too much about precision in recipes and following recipes, to the detriment of their cooking. For the sorts of things you're going to use a 'cup' for, being 20% off (Imperial vs. US) is probably fine; obviously more so the more ingredients there are in the recipe that are measured with cups.