Know Places I Can Get Cool Cheap Room Decor?

I'm not familar with the UK but I would try thrift shops. We can find a lot of things if you have the patience to shop around. Also try ads in the newspaper and on line.

Thift stores for Salvation Army and Goodwill are a must to get an idea what is out there for how much. Also, go garage sale-ing on weekends. I assume you have some idea what he had before if he is good friend, so you use that knowledge to know what items to look for and what kind of colors he likes. Is he a person who keeps his place at least picked up? He is a neat freak? Does he have any sense of taste at all? Do you know of any hobbies? Does his hobbies take up a lot of room?

Lining paper and stencils are good for making your own wallpaper. Charity shops and car boot sales are great for cool stuff to hang on the walls or make pictures by stretching fabric over canvas frames