When I did it I just took the exams without revising (it is possible to get a Gold this way). I wouldn't take it too seriously, just turn up and see what you can do.
However, there are books publish by UKMT that would help. I have some now, and they would have been really useful when I was doing maths challenges. I also would recommend the Art of Problem solving wiki... focus on learning a few techniques (modular arithmetic is invaluable for almost all the number theory questions). Also it would be a good idea to do some geometry (I absolutely hate it, but geometry tends to be ignored after primary school).