Is this something unique to British-brewed beers? According to https://en.wikipedia.org/wiki/Isinglass "Isinglass finings are widely used as a processing aid in the British brewing industry to accelerate the fining, or clarification, of beer". I cannot recall ever seeing anything like this on the ingredients lists of cans from other countries (e.g. French or Spanish beers), but maybe it doesn't count as an ingredients for some reason or maybe other countries use something worse to clear the beer.