If its ripstop nylon you're after, then you really need to find out where your nearest kite store is, they generally stock it by the roll and/or will know where you can get some. Its also quite cheap by the metre! They may even have some offcuts! I know my local shop here in Glos used to!
As i dont know what area of the uk you're in i dont know where your nearest onw would be.
Try an online search or yellow pages.
The only other thing i can suggest is if you have a local craft/scrap resource store (usually frequented by teachers) then check them out!
Posted:There's an online place called Turpin Kites or something like that, search under "Turpin" and "kites" and you'll find it. They do loads of colours in loads of different weights and will give good advice as to what weight you need.
There is a slight possibility that I am not actually right all of the time.