Whilst there are elements of ground school and the ongoing academic side that would be easier with a degree in engineering, physics, aero-eng etc it doesn't add up to that much. What the RAF are looking for is someone who can absorb and cope with information at approximately degree level. Therefore it doesn't matter what your degree is in, I did geography, I know people with marine biology, English, classics, accounting - it really doesn't matter, best preparing for your alternative non-military option or simply something you will enjoy doing and pass after three years.