I've used VetUK for years now, in the past I've bought prescription meds which have saved me a fortune and now I buy Frontline and Drontal wormer (no prescription needed for either of those), plaque off, logic gel etc
As CC says the Frontline are much cheaper, the Drontal worming tablets are around £1.50 each. Pets at Home is also authorised to sell veterinary strength meds now and they, for example, sell Drontal at 2 for £8.09
Delivery is free with VetUK (when you spend over £19) and they usually despatch the same or next working day using first class post so superfast too. They also donate 5% of your order total if you click through a rescue link to whichever rescue you've 'clicked through', occasionally you can find some items cheaper at other online pharmacies but in general (for the products I've bought) they're the cheapest so not only do I save money but I get to help the rescue out too
If you do order from them below is a list of rescues on Purrs that have VetUK links
If anyone orders from vetuk, and lets face it you'd be crazy not to given their prices and free superfast p&p, please order through a rescue link so they will get 5% of your order value donated to them by vetuk.
The rescues on here that have a link are below, I always take it in turns every time I order.
http://www.pawsinncatrescue.co.uk/ - click on vetuk logo on left hand side of home page
http://www.coventrycatgroup.org.uk/ - click on 'help us' tab, then 'shop online and help us' then click on VetUK link itself.