Is soon likely to increase because the other aircraft can't...
Unmanned spy planes to police Britain - Home News, UK - The Independent
Includes an interesting quote....
"UAVs are currently restricted to military installations in Salisbury Plain because of regulations banning them from using the same airspace as civil aircraft. However, a commercial consortium led by BAE Systems will provide the safety measures necessary for the planes to fly over the UK within three years"
So, one can only presume that it's BAe Systems, and not the CAA, that is pushing Mode-S /interoperability measures through currently.