The only things which might stop me from buying patterns would be (1) the exchange rate, and (2) the thought that I'd have to carry them home. Otherwise, some British designers have some nice patterns. Many of the British yarns are available in the U.S. now, too, so I don't know of anything you should snatch up - unless there's some great hand-spun yarn you stumble across. That's unique anywhere.
But if you like yarn stores, it makes perfect sense to do something you enjoy while you're on your trip.