Hi Claire, Glad that you enjoy the site... I enjoy writing it too!
With regards to plants for your green shed roof I would recommend that you go for a selection of the many varieties of sedum, semper-vivum, grasses, mosses, alpines and even some flowering herbs.
The roof environment is very different from that at ground level. The moisture level in the soil can go up and down very quickly. The plants that you use will need to be quite tolerant of lack of water.
I recently wrote up my experience of growing plants for a green shed roof from cuttings and plants that I found. This should give you some extra knowledge over and above that provided by your local garden centre.
Let me know how you get on it would be great to see some pictures of your roof as it progresses.