Ordered from the states, as there does seem no one this side of the pond selling it...
7/64" AMSTEEL BLUE - Seattle Marine
at $0.15 per foot, 600' = $90, now we all know thats only about £45 so even with shipping you should get under the tax barrier too.
Contact the site i guess before order to make sure its all in stock and the shipping rates to here. Plus you could try haggle on the price for a ful reel!