hyperdrug or pet sence direct, both have free delivery, Im due to order now and will be doing so from hyperdrug. I can check what I ordered the last time as it stores previous orders in my account.
I've used Nutrecare for my last few wormers. Great prices and delivered pretty quickly. Used wormers direct before them but found nutrecare to be slightly cheaper. No p&p either which is a bonus.
You get a referall code when you join, you can then pass this to friends and if they order using your code you get a discount off your own next order i.e if you refer enough friends you may be paying next to nothing for your own wormer!
Each wormer website has different deals at different times of the year & on different wormers. I think you have to check as many sites as possible to get the best deal. I've got good deals on Hadrian Equine in addition to the others mentioned above, but that doesn't mean that they are the cheapest at the moment. http://www.hadrianequine.co.uk/horse-wormers-1-c.asp