Should have added you can also adjust based on budget. Some of the best known brands score very poorly for dog nutrition.
Also lots of good advice on diet, allergies etc. Finally if you change a dog's diet expect results at the other end. You need to allow a few days for things to adjust and...
https://www.allaboutdogfood.co.uk/
This is a brilliant resource. Work out what is best for your dog based on breed, weight and age.
Petsathome, zooplus and bitiba are all good for cheap bulk buys.