Behind the UK's dog-friendly pub culture is a specific, fairly simple food hygiene principle: a business owner can choose to allow dogs in customer-facing dining or bar areas, but dogs are never permitted in areas where food is prepared, handled or stored, including kitchens and storage rooms, regardless of the venue's general dog policy.
Assistance dogs are the one standing exception to any discretionary rule — they must be granted access to all food premises, including kitchens and food preparation areas where a pet dog would never be allowed, as part of UK disability access law.
On the ground, this translates into practical etiquette: keep your dog on a short lead in the dining or bar area, off furniture and out of walkways, and expect staff to avoid direct contact with your dog and to clean up immediately and thoroughly if there's any accident — all standard local-authority food safety guidance for dog-friendly venues.