Is it possible to complete a PhD program early to leave it with a masters? I'm extremely unsure of what route I want to take in life. 

I desire to study at a top UK university, studying earth science. I've found a PhD course which sounds like a dream (it teaches all the skills I would wish for). No one in my family/ancestry has attended in university (I come from a long line of farmers from a developing country), never mind a top institution. While this gives me the desire to apply for graduate studies, it leaves me unsure of my capabilities or what possibilities exist within graduate school.

I'm uncertain if I'm cut out for a PhD. Originally I was only after a masters, but this PhD seems more appealing in terms of skills taught than everything else I have seen.

If I were to take an earth sci PhD at a top UK uni and decide it isn't for me, is it possible to complete it early and leave with a masters?

Further Details

I wish to study at graduate level at a top uni.

I've decided to go for a 1-yr masters to play it safe, as I'm unsure of my abilities. However, masters all seem to either be one long dissertation or a dissertation alongside lectures/field trips. I would love to also learn "hard skills" related to lab work, programming, etc...  

Found a earth sci PhD that teaches programming, thesis writing, maths, lectures across multiple departments, earth science overview, project planning. A well rounded course! I would love to take this course, but as mentioned, have no idea if I'm capable of composing dissertation level research/thesis/publications.

(I've struggled to figure out if you are able to drop out after 1/2 years and receive a masters of sorts, but here is the link to the course)
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.