The major difference between the two is the colour vision standard - or rather, the testing.
RN CP2 and the RAF's highest standard (CP = colour perception) are assessed by passing the Ishihara plate test, but RN CP1 (aircrew/warfare officers) is tested using Holmes-Wright lantern at half-intensity, in order to comply with Board of Trade watchkeeping officers' standards.
In reality, if you pass Ishihara, you'll almost certainly get CP1 as well.