According to CAA rules UAV's can only be operated outside controlled airspace if they can sense other traffic and be sensed by other traffic.
I have always believed that this is what Mode S is really all about.
Light UAVs can only be operated up to 50M from the operator and must remain in sight, just like model aeroplanes.
http://www.caa.co.uk/docs/1416/srg_s...-01-180604.pdf