This is a problem a lot of student visa applicants are encountering since the end of the Brexit transition period. The medical letter process is relatively simple procedure but is often performed incorrectly. People are making the mistake of getting doctors medical letter notarised and then an FCDO Apostille. This will not work for the Spanish consulate because they need the doctors signature and GMC registration to be apostilled, not the notary.
We will need to submit the letter to the FCDO who will check the doctor’s status and then verify they are legitimate by including their name and status on the apostille certificate. A word of warning post Brexit and in this covid age very few doctors have their details and signatures lodged with the FCDO, this means most applications are being rejected. Help is at hand as we can guide you and your doctor through this process making sure you don't fall foul of the Spanish authorities.