Northeast-timber on ebay may be worth looking at. Had a few good pieces from them before - but delivery is a little expensive.

You could also try Oak Leaf Creations at Nottingham if you can get there. Mostly non-planed planks and waney edge up-to about 3m in length. The stuff I bought was about 8ft in length and between 9 and 12 inch in width. And somewhere around 1.5+ inch thick. The stuff looks at bit shonky at first, but it planes up great. The guy also has a great workshop so he may cut to spec if you ask.