Pets at Home is by far the largest pet retail chain in the UK, with stores in most towns of any real size, making it the most reliable default option for restocking food, bedding, leads, or basic supplies wherever your trip takes you. Many larger stores also host an in-house Vets4Pets veterinary practice, which can be useful for a routine check or minor issue without needing to find a separate, unfamiliar vet.
Beyond Pets at Home, most UK towns also have at least one independent pet shop, and general supermarkets stock a reasonable everyday range of pet food and basic supplies for a quick top-up without a special trip.
For specialist diets, specific prescription food, or less common brands, it's still worth checking stock or calling ahead, since even the largest chain store won't carry every brand or formulation — particularly anything imported or niche to your pet's home country.