www.thetrustypawsclinic.co.uk/
Started in Glasgow and now expanding to London, Trusty Paws is run by fourth year vet students on a monthly basis, with clinics conducted at a homeless charity. They offer free chipping, vaccinations and worming treatments, as well as general health checkups for dogs owned by homeless people. If further treatment is needed, the dogs are referred to a local vet and bills paid by the charity. Donations can be made via their website.
(I have no connection with any of this, but saw an article in the paper and thought I'd pass it on.)