It is usual nowadays that everyone registers as Mphil/PhD in the first instance and undergoes some kind of formal transfer to the PhD route after 12-18 months work. The transfer stage is a useful step on the path to PhD and also, the Mphil/PhD route allows a student to complete the Mphil if they choose not to pursue a PhD (or have the decision made for them). However, institutions do differ in their policies so if this causing you concern, I'd advise clarifying the situation with the postgrad office. Good luck with your research degree.