I'm not sure about in the UK, but here in the US, I get mine from RO dispensers that are usually found in front of supermarkets. I can get 5 gallons for $1. Also, my LFS sells RO. You can maybe call around the LFS's by you.
If no one chimes in here, one of our AA members, sk3lly, is from the UK. Maybe you can PM him. You can find him in the planted tanks sub-forum.