The Green Mule (online) sell the 10kg bag for £10.99
http://www.greenmule.co.uk/pet-supplies/pet/small-animal-food/burgess/supa-rabbit-excel-pellets.htm
I've never used them so can't personally recomend them.
Berriewood Wolesale, also online, sell it for £11.88
http://www.berriewoodwholesale.co.uk/pages/burgess-supa-feeds-751.htm
WARNING, BERRIEWOOD SELL SMALL ANIMALS (RATS MICE AND CHICKS) FOR SNAKE CONSUMPTION... just thought I'd let you know incase you were against this. Having said that, I have used them and the service was excellent.