Best wishes for the future!
I suspect it is best to ditch the dreadlocks by the interview as you will need to convey the message that you are committed 100% from the start. If this is a problem, you may need to think about your reasons for joining at this particular time - you can always do the degree, keep the long hair and join later and this certainly would not be held against you. There are standards and conformities expected in the RAF, this isn't a value judgement as such but just the way things are. I wouldn't advise joining half-heartedly or on a try-it-and-see basis, as the responsibilities demand a full commitment.