Are you basing that on one experience in a Glaswegian shopping centre fast food outlet? I eat out once or twice a week in the UK and can't remember the last time something I wanted from the menu wasn't available.
No actually I am not. As a former Glaswegian, I have had my fair share of similar events. More so than any other city I have visited (with one notable exception in Florida where the full service restaurants were out of the first 2 choices I made. After being told they were out of both choice 1 and 2 I specifically asked if there were other entrees they were out of. The answer was firm..all else on the menu is available. I ordered liver and onions and they exclaimed "Oh, no one ever orders that". I just walked out at that point.