There are some very good and reputable online suppliers with UK addresses, run by vets, don't sell POM without a prescription from the vet etc., but always read reviews before using one.
I use Pet Drugs Online for William's Optimmune ointment (for Dry Eye) and have every confidence in them. I pay £12 for a repeat prescription from the vet every 6 months and can then buy Optimmune for half the price than the vet can supply it.