Whereabouts in the country are you? There's a few big suppliers but some have set delivery routes
E.g. DAF (mostly Midlands upwards but they have stockists nationwide) http://www.daf-petfood.co.uk/content/1-delivery
There's also the more commercial products like Prize Choice (which is also stocked in many petshops if you can collect locally) http://prizechoice.co.uk/
Probably missed out a few smaller companies in certain areas, e.g. I know there's a few smaller stockists in Scotland that I can't recall the name of (e.g. one in Dundee, one up in the Highlands that's just started up). Plus some butchers have started making their own pet mince - though this can be a bit unbalanced as it's often just all their waste meat/fat/bones minced up rather than a good variety in balanced quantities.
Good advice and list by krlyr, I have used landywoods for the last 5-6 years or so and have used the dogfoodcompany on occasion, nice friendly service from both but unfortunately none of my dogs liked thrdogfoodcompanies mince, it was a little more expensive than landywoods but I gave it a go to give the dog's a change.After a dismal "failed to start" with their mince, I ended up having to give it away and I went back to landywoods as they love their mince. This is only my own experience with 3 dogs and other peoples dogs seem to get on with it, maybe mine were so used to landywoods they didn't like changing, Oz
ps I think you'll find on average landywoods have the best prices and good delivery prices