One of the best camera backpacks ever made is the Domke Armadillo 30. There are a few new ones currently on sale in the US and UK, even though Domke discontinued them years ago after they was bought out by Tiffen. You can still get the yellow insert kits for them from Kata too. Since you live in the UK check out the link below, £79.99 is a steal for this backpack.
Domke Armadillo 30 Backpack Black from Warehouse Express