I came upon those two UK sites when looking for a euro source for silnylon, they didn't have it at the time, if I remeber right. Anyway, you can order from a US dealer. You just pay more for shipping but with the favourable exchange rate at present it'll be probably cheaper than anything you could buy at home, particularly if you are not charged customs tax (I sometimes am, sometimes not...).
I've purchased silnylon from Noah Lamport at www.noahlamport.com , they seem to be the best priced but you have to get a minimum of 10 yds. Actually, they don't call it silnylon but look under "Nylon Ripstop 2nds, Silicone Coated, Zero Porosity" and that is the stuff.
Then, there are many others. Off the top of my head:
Ayce at thruhiker.com might ship to you too.