ImpossibleIsNothing:
Although at Stage 1 your attire doesn't matter, I think you'll find yourself more comfortable wearing a shirt, trousers and shoes (no tie/jacket), as the other applicants will be in a similar style of dress.
You don't want to stand out as the guy who wore jeans/t-shirt/trainers at the Stage 1...!
But you'll probably be told to take off your ties and jackets when you get in the room (our group was!)