I asked a similar question recently, and the general response was as long as you have a bmi (body mass index) in the healthy weight range (20-25) you won't have a problem as long as there are no other bad health indicators. I am slightly overweight which is why I asked the question and was told even if overweight should be ok for airlines. However RAF are going to have much stricter guidelines so you will need to make sure you are very fit and healthy. Hope it helps!