I normally get mine through Hadrian. They will match prices, I've done it before when I found cheaper elsewhere. I order job lots for ours as we have a few to do!!
I think you just have to search because different companys have different offers. Sometimes hadrian have the best deal but other times wormers direct might be better priced. Sometimes the prices are the same but one company chargesd postage & the other doesn't. It also depends on the particular wormer you want.
Haven't used Wessex animal health, but have previously bought all our wormers from wormers-direct and been really pleased, they send everything out ASAP and are very helpful if you ever have any questions.