A World Atlas for Climatology.
CRP-5 is a must.
A non-alphanumeric non-programmable scientific calculator would also be handy.
Subscription to the Online Question Database - although it only lasts for 3 months so maybe better off closer to the first exams.
A school geometry set. Although mine cost me £1.49 in Tesco so hardly going to break the bank.
A magnifying glass for those horrible graphs in Performance, but you can't take it to the exam so ignore this item.
Lots of paper. A waste paper bin next to your desk. A few large A4 ring binders, you'll need at least one for the feedback at the brush up course.
Lots of pens... or a lot of refills.
That'll take care of the groundschool side of things.