Generally it"s common sense stuff - no ripped jeans, trainers, hoodies etc.
This kind of info is normally in the briefing issued prior to the event, which should be posted on this forum within the next 7 days.
If you can"t wait, then you could always search the forum for a briefing to a previous National event, this will at least give you some idea. Apologies that I haven"t posted a link here.