To become a Biomedical Scientist (provided it hasn't changed since I did the portfolio) you will need to have completed an IBMS accredited Biomedical Science Degree, you can contact the IBMS to find out about more about this if your degree isn't accredited and how you can do potential top up modules. You then need to complete the IBMS training portfolio in an IBMS approved training laboratory (some people do this as a sandwich year during their degree). This allows you to register with the HCPC. Once registered you can typically work as a band 5 BMS, from there you can specialise with other portfolios and training to move through specialist and senior roles. There's more about it on the IBMS website.