If I complete a BSc degree, can I then convert it to an MPhys by adding an extra year?
No, it is not possible to convert a BSc to an MPhys simply by adding an extra year. So, if you complete the BSc programme but want a masters qualification then you would have to apply for a postgraduate masters degree, such as a one-year MSc course.
Similarly, students who are on the MPhys programme are not awarded a bachelors degree (e.g. BSc) at the end of the third year followed by a masters degree (MPhys) the following year.