Obicat, you are so right. You do have to be creative. Lucky you to be in the UK! Your post reminded me. Our local transfer station (aka: the dump) has started having a day when you can come and get things other people want to throw away. I am thinking that might be a good source for lumber...
LOL! It was a blessing at the time, but it was not a pretty sight as my husband built it alone and had no carpentry skills at the time. That is why pre built walls were such a blessing. It wouldn't have been quite so bad if it hadn't been so tall. It really stuck out like a sore thumb. But...
We were able to build a coop and goat house for almost free one time. But that was only because one of my husband's customers had a whole stack of 4 x 8 framed out walls that he ended up not using and decided he just wanted them gone as they had been sitting around his property for a while...