Instructions:Just now I have found out that you can use promo codes from local Photobox stores in the UK store as well. German Photobox does have a Buy one get one free Classic A4 Photobook offer valid until 5. 8. 2009.
It says in German: "1 Fotobuch GRATIS Bei Bestellung eines Weiteren"
Moreover Photobox just lowered their price for these photobooks to £18.39 which makes it £23.38 for 2 Classic A4 Photobooks delivered!!!