MEDICAL
Doctors: As soon as you settle in the UK, and have a fixed address, you will go find your local doctor and register with them.
https://www.nhs.uk/using-the-nhs/nhs-services/gps/how-to-register-with-a-gp-practice/
Everything is NHS of course, so no money changes hands, except
for scripts where you pay about £9 per item.
So, if you require more than 2 prescribed meds a month, then
check out Prepayment Certificates: https://www.nhsbsa.nhs.uk/help-nhs-prescription-costs/prescription-prepayment-certificates-ppcs
Dentists, similarly, you register with them, but not so free.