Best reputable supplier would be the Canine Chemists http://www.hyperdrug.co.uk/ You will see at the top which section you want - equine, canine etc
If you are only ordering wormers, they won't charge you P&P and the delivery and service is great. Been using them for years, like a lot of CD'ers. :-)
I use http://www.vet-medic.com/ but do occasionally use Canine Chemists, most of the time though found Vet-Medic slightly cheaper and also quicker. At times I've ordered something one afternoon to have it arrive the next morning!
Thank you everyone. I used Hyperdrug in the end and just ordered the pooches wormers to try them out. Am very happy, I ordered them at the weekend and they arrived Tuesday. I will definately be using them again and ordering horse wormers and anything else I need from them.