GCSE revision books probably best, as there is no pilot's book (I'm trying to write one, but people keep wanting me to teach them things and I have not had the time!). The level should be correct.
You need reasonable trigonometry, to be able to rearrange a formula with three or four terms (R x A = 60 x D for example) to find one of the terms. You need pythagoras's theorem and a basic understanding of geometry of circles and of triangles. Ratios and percentages are vital, fractions useful. You will need to be able to interpolate figures that change depending on two independent variables (e.g. to work out sunset times that depend on latitude and on date, but are only tabulated every 2 degrees and every three days), but you will be shown how to do so. Use of standard index form (e.g. 2.3 x 10^12 = 2,300,000,000,000) would be handy. Some mental arithmetic will be useful for the groundschool and even more for the flying.
If you want a simple test to give you an idea send me a private message with an email address. I teach general nav, the most mathematical subject, for BCFT.