There aren't decent PPL study guides online at PPL level.
There are a number of sets of test questions - but that isn't what you are looking for.
The best text books are by Jeremy M Pratt or Trevor Thom.
You aren't gong to get a whole set for £20 but worth looking on eBay / Amazon market place as you will get them far below the new price, and since they are lots of individual books, you could build up the set slowly. That is, unless you know someone who has finished with their set...
Airlaw is probably the only one where you need the most up to date text book - and even then, both versions are currently out of date due to the EASA changes.
RR