I wouldn't worry about shirt and tie. Just go smart/casual. I went in jeans and shirt and was told even that was a bit too smart.
Obviousy don't turn up in trackys just be clean and have a good attitude and you'll be fine.
Every single time I have been in the presence of a member of HM Forces while applying for this job, apart from the time on selection in trackie bottoms, I have had black shoes, black trousers, shirt and tie.