The Oxford Learners' Dictionary says:
(also Mr. North American English, British English)
While the Oxford Learner's Dictionary of American English just states:
and has no entry for Mr without the period.
So I believe that both Mr and Mr. are acceptable in British English (even though Mr without period is preferred), while Mr. with period is mandatory in American English.
As an Italian, I prefer Mr without the period, because in school we were taught that the period following an abbreviation stood for "following letters omitted", but the r in Mr and in Dr is the last letter of Mister and Doctor, so no letter following "r" is omitted.
I think that the British usage tends to conform to this "European" (Latin?) rule.