The egg-ish Ecocapsule from Nice Architects is technically a 'portable house' with off-grid capability but would be great for shedworking. As well as a desk, features include a kitchenette, lavatory, and shower. The whole thing is powered by a built-in wind turbine and solar cells which cover its exterior. Should be available at the start of 2016.
Cabin Master: garden offices and studios to fit any size garden. Top quality contemporary or traditional buildings.