For the next Olympics...
Before a sortie that is planned to be within x miles or x flight minutes of London, police or private security (god forbid) based at every airfield inspect and give each aircraft a code.
The code is generated using a device similar to the $5 code generators that Barclays gives to its online customers. The code would be generated by inputting aircraft registration the days date and time of day.
The code is checked before startup with the tower who has a "reader" who ensures it matches the aircraft taking off.
When airborn, the code is transmitted by radio to ATC on request, who would enter the 4 digit code into their "reader", which would confirm if the aircraft has been security screened to be airborn at the particular time of day.
Helicopters conducting frequent media or ferry sorties who land at secure sites need not be physically inspected for each flight but still given a code that is valid for the next four hours.
It is established technology that works, and needs sod all training to implement and no hardware required by aircrew.
Other advantages;
The presence of police at
every airfield within a tank of gas range of London is a deterrent.
Since every crook knows about squawking 7500, the code could also be used as a basis to communicate hijack or duress.
After the Olympics it could also be used to allow easy entry into exclusion zones by authorised aircraft
Mickjoebill. daydreaming again!