Paying for your medical in advance, imo would be a waste of money if considering nats. They pay for their own. Even if you had a class3 medical, you will likely end up doing it again. On top of this Nats for initial entrants have varying requirements that are different to the class 3 medical ..eg ..sight test. previous thread on this. Check with the med team at srg and swanwick, if you have any doubts. Good luck