What's really fascinating about this discussion is the clearly deeply
felt need for there to be distinctions between what are nothing more
than arbitrary alternatives.  There have also been one of two
suggestions as to British usage which don't ring true.

The current British (or at least suburban London) usage would probably
split equally between merry-go-round and roundabout.  My much-worked
Oxford English Dictionary has merry-go-round in use by 1729, and this
particular use of roundabout by 1763.  The terms are quite
interchangeable.

As anyone who's driven a car here will know, "roundabout" is normally
what ordinary folks call gyratory traffic systems, which may possibly
have unnecessarily excited visiting fairground fans.  On the other
hand, "merry-go-rounds" are always at the fairground.  Only
train-spotters will think of the railway's coal-distribution system of
that name, where the train is loaded and emptied on the move so never
stops on its run from pit to power-station and back.

No member of the public would have a clue as to what "gallopers" are --
this is very much an insiders' term-of-art.  I'd not be surprised if
the film "Carousel" was the first anyone here heard that term.  Neither
of these are in the 1940s-revised OED.

Julian Dyer, London.