well, You can buy from Amazon, that might be cheaper, though not always updated.
Otherwise, i always order books directly from publishing. So check google: for kaplan – Kaplan financial for BPP – BPP learning media.
There u can select your qualification and books …and order them. With BPP u can buy e-version of books, but they will have only 15% of priting allowance. Otherwise i usually order hard copies, but costs of delivery are quite high.