Hi there,
try hitting the search button and typing in NATS SELECTION. There's is this one thread that contains a website link all about the sort of questions u'll get and answers most initial problems too! V.Good.
Most people say you can't pratice for the test, u can either do it or not! but you will need to refresh yourself on Fractions, remembering numbers and letters, and basic aviation knowledge, which you probably have.
Nats reckon you should pratice crosswords and puzzles, i think the best way to pratice is to invest in a good RAF Amptitude book and work through the questions, so in answer to your RAF bit, a quick fresher and you should be there!
good luck