> Which sites are best
Go for a reputable, well established, and well known pharmaceutical company.
I use
http://www.hyperdrug.co.uk/I buy all my Panacur 10% for worming pups, at a great saving.
Remember that there will be prescription only medicines that you can buy too. You don't need a prescription for wormer, and, if it's all you're ordering, hyperdrug don't charge P&P for wormers. If you want prescription drugs, you just ask your vet for the prescription, telling him that you have found a cheaper source.