Originally Posted by
stut
This always used to be the case, but is rarely so now. A couple of booking sites do still use it, as does the kind of B&B or guesthouse that caters to the kind of person who feigns ignorance of metric measurements, is constantly making jokes about "Health and Safety", and, every time they say "Happy Christmas", follow it up with "if I'm still allowed to say that!"
Going slightly off at a tangent, I am always surprised how sparse information is on hotel websites about bed types, sizes, room types etc. It is often difficult to understand the differences between deluxe, superior, classic etc.
Back on topic, my expectation in the UK (and Europe too) is that double is a double bed (4ft6 or wider) and a twin is two singles.