Good advice above. Learning the gas laws in general, basic electrics, and revise some simple algebra, and trig.
Covering the PPL subject material would help, but it would be a waste of money for you to buy PPL study books. If you can borrow the technical subjects, including navigation, it would be helpful for you.
As a reasonable background to the technical subjects, you might want to consider reading A.C. Kermodes Mecchanics of Flight.
Purchasing the Oxford CD for Met would be a good investment too.