check out craigslist.org (click on your city name)
theres a section for pets..

and also petfinder.org

bukpets.co.uk/animalcharities.htm
Animal Charities, Pet Rescue Centres, Pets free to good homes, give donations, foster homes and more in UK

bukpets.co.uk/dogs.htm
Terriers, German Shepherd, Labrador, Lurcher, Siberian Husky and more in UK

and so on for relevant pet you are looking to rehome or wishing to give a new loving home to.