I used a pair of altura nightvision trousers last year. Super light, super packable.
Pretty thin, but really waterproof. Would wear shorts underneath, but you could wear trousers if you wanted. Depends on the intensity of your commute I guess. Mine was pretty high intensity, so never got cold with a thin pair of shorts. Think they were pretty cheap off wiggle too.
Lots of adjustment too, including length, and straps to stop it getting caught in your chain. They don’t look wicked cool or anything, but they do work very well. Reflective strips too.
EDIT: £52 http://www.wiggle.co.uk/altura-night-vision-waterproof-overtrousers/
😯 don’t remember them being that expensive..