
I may be wrong but the only genuine multi-dog policy I'm aware of (if it's still done, it may not be) is from Healthy Pets. I had 3 dogs with them years ago but I left after the premium doubled after no claims and back then, all young, healthy dogs.
Some companies offer discounts for more than one dog though - Animal Friends do, Petplan I think does but not 100% sure on that. Others too I think but can't think of names.