Shop own brands can be just as good as some well known ones, most are made by a company called Golden Acres up in Preston (who make for Burns, Arden Grange and until recently I think, James Wellbeloved, though I believe Royal Canin make this at a new factory now) - GA have a recipe book and you just pick one out and they'll make a batch for a shop. They're based upon standard recipes for chicken and rice etc. and although not often what you'd call premium foods they're probably equivalent to a medium priced branded pet food.
The big example of shop own brand is Pets at Home who have a huge range of own branded pet food these days.
If you want more info on saving a few £s then try this link Pet Food Choice - Money saving tips