I was going to recommend decathlon as well. I just bought the mens rain jacket for about £18 and is good, although hasn't been used yet. If looking strictly for one that can be stuffed into your back pocket, the one I got would need a pretty large back pocket. I keep mine rolled up in a sawn off drink bottle in one of my bottle cages.
Also mention that the first one I got was too small, but their returns process was so easy.