I had already, wrongly assumed that they were equivalent
It's not so straightforward as saying that they aren't equal, because you have the headache of comparing pre and post Bologna magister degrees too. Probably the only real, fair way to compare the two is to compare what was studied (and the amount of hours) for each module - some Polish degrees are ridiculously soft, whereas some are much harder. The one thing that really lets the Polish system down is the way that 5th year modules can actually be easier than 1st year modules - which is clearly nonsense. The same nonsense is seen in primary and secondary education.
The answer that I was after was regarding the use of the pre-nominal title "Mgr" and specifically, whether the use of the title was restricted to those who had achieved a Polish Magister qualification, or whether a holder of a UK Master's degree, as an equivalent (or better) qualification, could aptly use the title on, for instance, a CV or business card.
Stay away from doing it. There's just no need - MSc is known and recognised, and it allows you to make it clear that you studied somewhere in the English speaking world. Whether or not all Magister qualifications are inferior could be argued about all day - my own opinion is that some are equal, some aren't - but it is certain that it would be stupid to say that your degree is one of them when it clearly isn't.