Twilkes
Guru
No idea if this has come up before, but £20 for a light waterproof jacket can't really be beaten - probably wouldn't take a whole day in the rain, but for commuting it's ideal. Windproof & breathable, only downside I've found is that the hood isn't detachable, but it has some reflective strips on it, pockets, some armpit vents for the terminally sweaty, and a little coat tail to cover your waistband while leaning forwards:
http://www.nevisport.com/pr/5467/insky-waterproof-cycling-jacket-mens
http://www.nevisport.com/pr/5467/insky-waterproof-cycling-jacket-mens