These days it is not necessary to buy prescription only veterinary medicines (POMs) only through your vet as there are several reputable online retailers. The Veterinary Medicines Directorate has been promoting their Accredited Internet Retailer Scheme to those who buy or are considering buying medicines online. The Directorate recommends that you look for the scheme logo on websites in order to give you reassurance that you are buying safe and effective medicines.
The VMD has produce a leaflet about the scheme which can be seen on their website:
You may also find a particular site through personal recommendation. Just remember to check if there are postage and packaging charges to add.