The Porterhouse address this issue too. They have two beers which are not suitable for Vegetarians: Oyster stout, which has oysters in it and TSB which is a cask conditioned ale and uses Isinglass.
I think that Diagio is overestimating the use of this stuff by other brewers too, by the way. I think they are effectively going "Well, everybody does it." It it is common, particularly in real ale, but far from ubiquitous. Any beer made according to the Reinheitsgebot, for example would not contain Isinglass.