Dress code: Personally I'd follow whatever NATS have said. Stage 1 used to be smart/casual (I'm assuming it still is), so I'd like to be seen to be able to follow instructions rather than showing up in a suit for the hell of it. Suits will be required later on when dress code is upgraded to smart for later stages.
Speed, distance, time. Rather than trying to do fractions in your head, you'll find it a lot easier to think in miles per minute rather than miles per hour. You'll also discover that the numbers in the questions are chosen so you can do it this way.
e.g. Aircraft flying at 420 knots. How far in 4 minutes?
420 knots = 7 nautical miles per minute. Distance travelled = 4 x 7 = 28 nautical miles. For me, that's easier than 420 x (1/15)...