As a serving member of the military - I'd thoroughly recommend getting the class one medical now.
Military medicals are quite different from an aviation medical, (You have to be fit to fight, not just to fly) but medical issues during application to the armed forces can be a complete pain. If you get a class one now you can at least have some confidence there are no major problems and even if something does show up, it's often a case of getting appropriate medical evidence from doctors that everything is OK. Getting that evidence now will mean you can simply give it to the RAF doctors when they ask for it without delaying things.