As far as I know, the vast majority of universities in the UK register their new students as MPhil students for the first year, after which they have some kind of transfer thesis/essay/viva, and only then are they registered as PhD students. Assuming this is the case with you, you would just have to show that you'd completed the current work satisfactorily, and instead of transferring to PhD, you'd just take your MPhil away with you. Perhaps it would be best to speak to your university's postgrad department about this though, in case it differs.