I think the way it works out, is that Customs gives it a value that they think they are worth in the UK, and if it is worth £18 (including postage) then you pay the duty on it. How much you pay is entirely up to them, but varies between 5% and 9%. The website I checked out said you have to pay VAT, but the official government says you don't, so I can't help you there.
Here are the sites:
http://www.hmce.gov.uk/public/shopping/shopping.htm
http://www.sloanefox.freeserve.co.uk/importukduty.htm
If you had a friend out there you could trust, you could probably get round the Customs charge, People are allowed to send presents into the country (upto £36), without you having to pay the duty/VAT. So if you bought a years supply for £100, and sent it to your friend in the States, he can split it up, and send it to you in 4 parts. He must clearly mark it a present though. Of course the Customs guy is at liberty to decide wether you're trying to cheat the system or not, and add duty/tax/fines so use that method at your own risk.
So basically, i'd advise you not do it. Sorry.