Why not look at a raspberry pi with xbmc installed? All apps are available and streams fine for me
Last time I checked Demand5 and 4oD don't work on XBMC due to them being behind DRM protection of sorts now.
Jez, some of the BD players have all those channels plus the ability to stream from network shares - plus have the added bonus of disc playback. Might be worth looking at too. I think Samsung, Sony and LG have the best content available.