Just wanted to know can you join the infantry in the RAF then train to be a Pilot when you gain in rank?

When you say RAF I'm uming you mean Royal Air Force. To be an Officer you need to complete college then the 32 week training to be commissioned. When you enlist you are enlisted, when you commission you are an officer. If you enlist as Infantry you would be in the Army because the Air Force does not have Infantry and also you would be working up in the ranks of a Non Commissioned Officer (NCO). You are thinking of the wrong branch and rank structure. In my experience, if you want to be an officer go for that only, do not waste your time as an enlisted airmen, soldier, sailor...whatever, it will only de-motivate you and you will get out before you reach your goals.