Here's a small collection from me.
2Excel Aviation PA-31 Navajo with Dusty Bin modification
During the British Army 'Pashtun Dawn/Tiger' exercises on Salisbury Plain, contractor operated ISTAR aircraft would overfly frequently. It was always most interesting to listen to the radio communication and target acquisition chatter on the scanner, using everyday events as simulated threats including on one occasion an old lady on a bicycle cycling through Chitterne who was subsequently targeted by a couple of Apaches.
A Diamond DA-42 MPP patrolling over the Wiltshire tribal areas
and Cobham Turbine Islander with the ISTAR pod just visible on the port side
This Airscan Cessna 337 Skymaster was operated on behalf of the US Government over Macedonia. Here it is seen at Dunkeswell in Devon minus the FLIR ball and other gadgets.
One of the much written about fleet of Cessna 406 Caravan IIs that operate over London and SE England.
Finally, I believe that Avdef are a Cobham company and operate in a similar way to the UK based Falcon 20s. The NATO JEWCS are based at RNAS Yeovilton so the Falcon 20s are occasional visitors.
Cessna