I don't know about the Magic, but I believe the Dream and Fantasy have both started vaccinated crew currently onboard. I believe that was in Port Canaveral (Dream) and Miami (Fantasy). Galveston is also preparing or started vaccinating ships' crew in port. Hopefully similar will be available to the Magic at a UK port.
When I saw the 18+ vaccination requirement, I guess I thought maybe the UK wasn't vaccinating the older teens like the US. I would hope fully vaccinated 16-17 yr olds can board without a test. I think you'll need to confirm with
DCL - and maybe closer to the cruise date will provide a more accurate reply. Sounds like a strong young lady, all the best to her!