"Pray tell me why it should be necessary to add this facility to the standard prefix list"

Funnily enough, I think people would be more likely to add "UK:" if it was there as a standard prefix. You might as well ask why have any prefixes at all... The psychology of it is that you are persuaded that you should use one of those prefixes: hence the number of times that a prefix is used when it really isn't appropriate, because there isn't one that fits but people think they should choose one anyway.

Anahata