Someone with the IOT paperwork can correct me if the wording has changed recently, but you guys have to understand this or you will embarass yourselves one day.
The default dress for any official dealings with the RAF is SUIT. That is jacket, matching trousers, leather shoes (polished), shirt and tie (windsor knot, tied right up to cover the collar button!).
Smart casual = casual in the eyes of the RAF. Smart casual would be Polo shirt and Chinos, with a thin woolen jumper, or blazer style jacket. Certainly not denim, no matter how expensive. As an prospective Officer, this would be the minimum dress I would appear at an AFCO in, and even then, only on a casual drop-in basis, to pick up literature etc, not a pre-booked meeting.
For Airman entry, the same rules really. You could turn up to everything in smart casual, however, if you wore a suit, you would set yourself apart from the rest in a positive manner. You might feel a plank if you are the only applicant in a group that turns up in a suit, but be assured, the AFCO staff will remember you for the right reasons.
Remember, the RAF is still a very traditional organisation in these respects. And one of the prime rules of this sort of society is that you can never be over-dressed (ie too formal). Formality shows respect, inappropriate informality shows disrespect in these circles.