There is already history of mid-air collisions involving manned and unmanned aircraft in busy bits of military airspace and several Airprox style reports from drones flying near civilian airports.
It's only a matter of time as the small civi multi-rotor systems get cheaper and proliferate. People with absolutely no 'air sense', as Flight Safety like to call it, are flying these things all over the place and the CAA is powerless to stop them.