Joe - you hit the nail on the head with first impressions count.
Whatever you wear, remember that this isn't just a video and coffee. It is your chance to make an initial impression. If you decide to go ahead and apply, chances are the people writing that first report on you after your initial interview - the one that can kill any chance of even getting to OASC, will be the ones running the presentation.
My advice, jeans and t-shirt are right out. I realise many Messes allow them all the time, mine does, but remember selection & training is a different beastie to the 'real' RAF. A suit would not necessarily be OTT; it would suggest you are serious about being there and as I said, this will be your initial impression. However, remember that there will be lots of you crammed into a small dark room and suit could be a bit warm. Nothing like looking smart but falling asleep!
A good compromise would be chinos and a smart shirt - day to day 'smart' standard dress for most Messes and more comfortable than a suit. If you dress as you would in a Mess, they can't criticise!