Now that is a new one on me. I had always thought that "mom" (with an "ah[r]" sound) was restricted to North America. One can learn so much from this forum if one is willing ! Incidentally the LPD confirms what you say, and gives the British pronunciation as with short "o" as in "dog", as one might expect. Oh, and apologies to Robbo for calling him a colonial when I should have known that he was a true Sunderlonial ...
** Phil.