Hi, the post have already mentioned most of the details joining the RAF as a BMS. However, a few pointers for you:
To to be a qualified entrant, you'll need to to gain the Certificate of Competence via the Institute of Biomedical Science (IBMS) and be HCPC registered. Therefore you must ensure that the degree you are undertaking is approved by the IBMS. Check on their website for your course.
If you do not have HCPC registration, you would enter as unqualified entrant.
Basic training is the same which is followed by a military BMS conversion course - this allows the RAF to teach you multi-disciplinary skills which are essential in haematology, microbiology, chemistry and transfusion.
Speak to the AFCO - they will be able to advise you on what the best course of action would be for you to apply.
If you can, get a visit in to find out what the trade is like in the UK and overseas.
Good luck