Hello mate, Just go casual, Jeans and T Shirt, Certainly not a Shirt and tie jobby, They just tell you most stuff you already know and put on a dvd for you to watch about the PRMC etc, Also gives you chance to chat to someone in the marines!. Also if they let you , you can get your application but some AFCO's are different to others most normally want you to go off and do some training and come back in 5 weeks or something then they give you an application.
Considering you've posted in the POC section, i'm assuming you are aiming to enter as an Officer-in which case I would suggest wearing smart shoes, smart trousers, a shirt and tie. It's important to make a good first impression and show that you are serious about a career in the Royal Marines, instead of looking as if you've wandered in off the street
Defo shirt and tie. you want to come over as prepared, professional and smart from the very outset, plus if you turn up in jeans and others are in suits you'll look a right *text deleted**text deleted* whereas turn up in a suit and others in jeans you can always get rid of jacket and tie and say you came straight from work (where you have a responsible position requiring maturity and ace leadership skills)!
If you go to the Armed Forces Careers Office and you are serious about joining as an Officer, wear a suit and tie or at least a blazer and tie with some smart trousers. You are making an application to join the Royal Marines as an Officer, you should treat the application process as you would any other job interview. I have seen POC candidates told to leave Lympstone instantly for not turning up in suit.