You can always buy iTunes cards from stores, I've seen them in Tesco and Asda, you can buy them from Apple store and many other places. You can buy different ones. £10,£15 and £25. Then you can buy as many apps as you wish. Its not only apps, you can buy musics, movies, books. I mean the card will bound to your apple account and you can spend your money on any store, even mac app store.