In my experience it is pretty common to see it used both ways.

I'm not from the UK though, and why they would eschew the simple-and-obvious m and b suffixes in favor of alternatives, especially when Europeans love to lecture us about how metric is so neat and intuitive.

Probably has something to do with the root words -- mil means 'thousand' in a lot of latin languages, and a thousand is M in roman numerals, and I heard somewhere that "MM" stands for "mil mil" or "million million" which, in the context of M equaling 1,000, woudl make sense.