Is it possible to complete a PhD program early to leave it with a masters?
Most U.K. universities don't give out MA/MS degrees to PhD applicants because most U.K. universities assume you already have one (or are an exceptional student). In contrast virtually every PhD program in the US will award you an MA/MS if you leave after two years. This is because there's no expectation of having a masters and so the first two years of a US PhD are pretty similar to a masters program.

Some UK universities will give you an MPhil if you leave early, but that's very different from a MA/MS degree. An MPhil is still an advanced research degree and if you split post-graduate degrees into "things that are approximately MA's" and "things that are approximately PhDs" it falls into the later category. Wikipedia says:


  The Master of Philosophy (abbr. M.Phil. or MPhil, sometimes Ph.M.; Latin Magister Philosophiae or Philosophiae Magister) is an advanced postgraduate research degree. The prerequisites required for a Master of Philosophy degree make it the most advanced research degree before the Doctor of Philosophy (Ph.D. or D.Phil.).1 An M.Phil. is in most cases thesis-only, and is regarded as a senior or second master's degree, standing between a taught Master's and a Ph.D.[2] An M.Phil. may be awarded to graduate students after completing several years of original research, but before the defence of a dissertation, and can serve as a provisional enrollment for a Ph.D.


As always, it depends on the specific policies of the university though.