Best deal I found over the weekend was Wessex Animal Health for 20 tubes of Pramox. www.wessexanimalhealth. But most online sellers will do a price match. There are loads of them. wormersdirect.co.uk are usually pretty good too.
Sorry to buck the trent here, but I think hyperdrug are by no way the cheapest.
I have worked out what I need for the whole of next year (wormers and suppliments) and I made a 20% saving by going to www.mileendequestrian.co.uk compared to hyperdrug!