Lynne Murphy is a linguist who has written a lot about the differences between British and American English. In 2012 she included
locum in a
list of "untranslatables", saying
BrE locum. Someone who stands in for someone else in a professional context, particularly doctor or clergy member. This is a shortened form of locum tenens, which one does see a bit in AmE medical jargon these days (but not just locum, and not in general use).
My own researches confirm this. The News on the Web corpus shows
locum usage in the UK at more than 13 times its frequency in the US. Moreover most of these US examples are from specialist medical publications, and many of them are used in the phrase
locum tenens.
I suspect the word is also declining in usage in Britain, Australia, etc, because of the disappearance of single-doctor medical practices, as people have mentioned.
Locum will continue to be classed as not common.