I turned up in Vans, Jeans, Textile Jacket, Gloves, Helmet and still got told off for not wearing appropriate footwear and normal jeans when I did my A2 Mod1. Like @iron_monkey said, its better to give a good impression of yourself, most of the instructors are strict on rules and safety...Legally i think a helmet is all thats required but i imagine if you turned up in t-shirt and trainers even if they would take you out they'd be looking for a reason to fail you.