Curiosity got the better of me - had a look at CAP 393 and, by my reading of it, think they were probably ok to skip this. Para 53(3) reads...
"In the case of an aircraft which is not a seaplane but is required by article 25(13) to carry cabin crew, the commander shall, subject to paragraph (9), take all reasonable steps to ensure that, before the aircraft takes off on a flight to which this article applies on which:
(a) it is intended to proceed beyond gliding distance from land; or
(b) in the event of any emergency occurring during the take-off or during the landing at the intended destination or any likely alternate destination it is reasonably possible that the aircraft would be forced to land onto water;
all passengers are given a demonstration of the method of use of the lifejackets required by or under this Order for the use of passengers."
So, on a flight to Glasgow I assume you wouldn't normally intend flying outwith gliding distance from land and - I wouldn't fancy ditching in the River Clyde on a 23 approach - would be near on impossible to do in any case!
Cheers for the thoughts folks...