I'd say you're better off getting a class 1 from the onset (not 1st class, it's not a degree
).
They check a lot of stuff, and somehow manage to make the initial last all day. You have a check up with the nurse where she weighs you and measure you, does an ECG, takes some blood and urine and any other fluids you may want to give
Then you see a doctor who makes you strip off and then he checks you physically, playing your rip cage like a xylophone and the "cough please" and all that.
Then the usual hearing tests, eyesight, colour blindness, and so on.
The RAF medical I'm sure will be more taxing still, the class 1 is for civil commercial aviation, and the two are in no way compatible or transferable. I know plenty of military guys who had to get a civvie medical, even though I'm sure their military one is far more strict and thorough.
If you have any thoughts of going commercial in the civvie world, better to get the class 1 from the word go, then you know you're safe. It is only valid for a year, but then reverts to class 2 which is all you need for private flying, and this is valid for 5 years.