The reason why
tourists find this incredibly
funny (I know because I've been one) is that the
gap passengers are warned about, through
announcements (made in a funny voice,too, that I unfortunately cannot
reproduce in writing)
and signs is in actuality often no more than a few
centimeters (or would that be
inches?) wide, something you would not be warned for in most other European countries. Thus it's a prime example of the British
habit to warn people for anything they might possibly hurt themselves with, including
steps up and down, low
ceilings and combinations of those. Mind your head, mind the step, mind the gap.
Where I come from people are just supposed to watch where they step.