There is no dress code , but I’d recommend smart casual to semi-formal. If you are learning an instrument through a school , check to see whether your school requires you to be in your school uniform. You wont be judged or marked on your appearance , but its a general sign of respect if you turn up well presented.
No , you dont need to complete a particular grade before moving onto the next. Having said that , by about 6th grade you will need to have completed particular grades in Musicianship or Music Theory to be able to recive your practical results. eg for Grade 6 practical , you are expected to have sat your grade 3 theory , Grade 7 practical requires grade 4 theory and so on.
There are no rules and this depends on your preparation and ability. On average its 1 exam per year , but some people can easily pull of 2 exams a year.