https://www.dogfoodadvisor.com/best-...ed-puppy-food/
This should help for basic guidelines,just apply the information from that source to UK products(or if you're able to get certain US foods that are available in the UK then that might work nicely).
The basics of any dog food and treats(almost all applies to any pet though,except in the case of some animals like hamsters which can have corn and wheat)would apply though,so no corn or wheat(terrible for carnivores,not only an issue of allergies though,keep in mind this appears in different forms like maize and wheat germ or even spelt),no artificial flavors or dyes,no maltodextrin/dextrose/sorbitol/sucrose/etc and similar processed sugars and artificial sweeteners,no vague biproducts or meats(digest,derivatives,meat and bone meal of any sort,vague liver,etc),no propylene glycol,no sulfur dioxide,nothing from China and nothing irradiated(no rawhide either if you don't already know since it is despite any manufacturer's claims completely indigestible).