If you’re looking for a dog-friendly pub in England, Scotland or Wales, I have a couple of resources for you. The first site, Doggie Pubs, is no-frills in both presentation and requirements: A pub’s food and drink have to be good, and your well-behaved dog has to be welcome to sit by you while you eat (it’s just not good enough if you can only have a beer with your dog).
Winalot (a Purina brand) worked with Doggie Pubs to create Winalot’s interactive map of dog-friendly pubs in the British Isles. It’s more glamorous and user-friendly than the Doggie Pubs site, but presumably contains similar information. (Please note that Winalot’s site also offers an interactive map of dog-friendly British hotels.)
If you prefer to flip through the pages of a guidebook, check out the Good Guide to Dog Friendly Pubs, Hotels and B&Bs (the current edition appears to be available only from amazon.co.uk). Brought to you by the same people that publish the popular Good Pub Guide, it has mixed reviews, with the main complaint being that it’s not comprehensive.