check out www.gumtree.co.uk they have lots of furniture and freebies too!
Ikea sells unpainted furniture, plus it's cheap. However, if you do not have one near you, the shipping prices can be outrageous. If it's not necessary that the furniture be unpainted, why not try a "clearance center" of a furniture store. I once got...
Use a sofa or long table to create a wall that way it separates the living room from the dining room.