Heathrow Director and forbesy64, the information about the three Islanders at RAF Northolt is out in the open domain - here is a quote from Wikipedia about RAF Northolt:
Current units at the station include No. 32 (The Royal) Squadron RAF, No. 600 Squadron RAF (Royal Auxiliary Air Force), 621 EOD Squadron RLC (part of 11 Explosive Ordnance Disposal Regiment), Royal Logistic Corps BFPO main sorting office, including the Defence Courier Service (DCS), both of which relocated from Mill Hill and the "Northolt Station Flight", flying three Britten-Norman Islanders. Their role is electronic intelligence gathering - described as 'embassy and anti-terrorist surveillance'.[4][5] Details of the Islanders; ZF573 Islander CC.2A,delivered to the unit in the mid-nineties. ZH536 Islander CC.2,the first aircraft acquired in late 1991. ZH537 Islander CC.2B,the third aircraft delivered in April 2008.This aircraft formerly served with the United Arab Emirates Air Force.
And even the RAF Northolt's own web pages describe the Islander duties
(
RAF Northolt - Aircraft and Equipment)
Talking about the OSA here is BS
And the GMP Islander changed from blue colours to a more military grey in 2005/2006
