You would think that having a Surrey postcode I would have no problem getting free wood but I live well within the M25 and have not yet found a regular supply so have bought some wood from Sunray Farm, 59 Woodmanstern Lane, Wallington, SM5 4BJ. He charges £6 for a dustbin full of hardwood less for softwood. You pick your own, the wood is chopped to a size easy to handle but too large to use as is. The wood is naturally seasoned and to my newbie eye some of it is a bit fresh.
Not an unreserved recommendation but I have not found cheaper and have had great success using wood from there. Also worth a visit if you require natural wood for other projects as the chap is very friendly and helpfull.
I have used wood from this site for both Pizza and roast and the only problem I have had is when I used some that was a little fresh while trying to light the fire, i cheated and got the blow torch out problem solved.