I know of one site where you can buy SF722 but it's based in America and I don't know what the conversion rates are, but by the time you have it shipped it to you, I'm not sure that you will find it cheaper than the 30 pounds you are talking about...
Check out http://www.modernherbalist.com
They ship internationally, but like I said, it might not be cheaper than what you have been finding in the UK.
Hope you find some if that doesn't work for you...