SJS cycles have both Dahon and Brompton allen key skewers (I know because I ordered both a couple of weeks ago). Neither are expensive. I ended up using the somewhat more compact Brompton skewer.
Couldn't see any difference in length between them.
http://www.sjscycles.co.uk/schmidt-f...ver-prod23926/
http://www.sjscycles.co.uk/schmidt-a...ubs-prod19064/