Do a google search on ctd tiles, they have outlets all over the UK. But, if you go in there in your work clothes, and keep saying, that price isn't low enough, they soon barter the price, especially if you mention topps is cheaper............
Its a bit of a trek for you but Tilemania in Hillington Industrial Estate do a really good discount on their tiles. Between 15% and 25% dependent on the range. There materials are very reasonable as well. They only do Dunlop adhesives and grout. I have found them very good to work with. Apparently the stuff is made in the same factory as Bal. All I needed was a business card to open my account