M. is Monsieur, and Brighton may have some francophones, so that's out.
Talking of which, the French government has now ordered that Mademoiselle and Mlle shall not appear on government forms. Married women don't use it and single ones don't like it. It equates to Miss in anglophone countries, some of which give the option of Ms; for 'manuscript', I suppose; for those who,like the French women, find Miss disagreeable.
Cost, aog? Surely the printing of Mx won't cost any more than printing two other letters in a sentence costs, assuming that, if adopted, the Council don't scrap all existing forms which lack it. It's not clear that having diversity training costs much either; what valuable work is not being done in the time allotted to it ? We have to give these civil servants something else to do, you know; the ones in our Council spend all their time passing bits of paper up and down corridors, to give the appearane of working while not having to make any decision !