In Pets At Home they have trial packs of Hills Science cat and dog food.
99p for 400g cat and kitten food
£1.99 for 1kg dog and puppy food
This is a really good offer as normally for a 3kg bag is £15.20.
*Edit* 3 per customer
Also if you sign up tp their club you get a £5.00 print out voucher. My local pets at home took this, although it does state to be used on 1.5kg bags or bigger. (luck as to how vigilant they are I guess).http://www.hillspet.co.uk/Adult/
Only avaible instore.