You might find it funny, but the best I can recommend is practising 11+ papers
. My sister sat hers as I took my aptitude tests and they had almost anything you will encounter on the NATS tests in them, bar the aviation specific stuff. As far as numerical and spatial reasoning can go I found them excellent.
As for books, a quick search of amazon revealed these-
Mechanical & Spatial Aptitude by Learning Express - "cubes"
The Aptitude Test Workbook: Over 400 Practice Questions by Jim Barrett
Air Traffic Control Test Prep by Patrick Mattson - I bought this one before I applied, it's quite expensive and not even based on UK ATC so don't take any technical info out of it. However it is a prep for the FAA ATC entry exam and has some really good spacial awareness and numerical memory exercises in there.
Good Luck
