I believe, as @Apthorpe said, that cherry plums are quite variable. Having said that, I also believe that the terms "mirabelum" (in various permutations of spelling) and "cherry plum" get used as a kind of catch-all to describe several kinds of small hedgerow plum.Too small, too dark colour and flesh too yellow in the OP pic to be Cherry plums Which are a brighter cherry red and have red flesh
IMHO
S