So that you are no longer alone let me answer some of your topics...

AC is GB

AC Shelby/AC Cobra should (probably) be named Shelby American (USA) ... some research needed.

Ford Falcon was not a German model, and I think not a British either.

For all the badge engineered cars (Daewoo sold as Chevrolet, etc.) the Origin is the place were it was produced. So A 'Chevrolet Matiz' would be Origin: South Korea -- Made for: EU.

Same for
Dodge Barreiros: Origin: Spain
Dodge Renault: Origin: United Kingdom
We don't want the owner (now DaimlerChrysler) but where it was produced.

Only if a production site is a plant of a company that just happens to be in a different country but has no roots there and no independent production, only then I would use the country of the main company. If a car is identifyable as being made in, lets say Australia, we should use that country as country of origin.

Specific Australian models are, for example:
Leyland Mini Moke
http://www.imcdb.org/vehicle_17917-Leyland-Mini-Moke-1968.html
http://www.imcdb.org/vehicle_16315-Leyland-Mini-Moke-1966.html
International Harvester of Australia (that's the full name)
http://www.imcdb.org/vehicle_11147-International-Harvester.html
http://www.imcdb.org/vehicle_11168-International-Harvester.html