How do universities in the UK define a distinction for a masters? I.e. is it based on GPA? (If so - what GPA?)And how important is it that you get a distinction in your masters when applying for a PhD? Thanks!

A distinction in a UK Masters degree will depend on how well you have performed in exams. In many cases greater weight will be given to the performance in your dissertation. Whether or not a distinction is imperative for admission to a PhD will depend on the subject area and the competition for places at that time.