Medical first - if you fail that, then your nascent aviation career is over. If you fail an assessment, you can do it again or follow a modular route.
A Clas 1 medical validity period depnds on your age - check out the CAA SRG website for the details. Last known cost was about £370 plus expenses of getting to Gatwick (where the initial has to be done).
You can always let your Class 1 lapse to a Class 2 for PPL purposes and then just renew to Class 1 standard (renewal criteria are not so stringent as initial).
Cheers
Whirls