This has always been the flaw in the plan. You may fly from a green list country but find others on the flight who have come from a red list country. The aircraft may have recently been on a red list route, and we know deep cleaning is not as comprehensive as claimed. Then you all mix in the terminal transit, then you pass each other several times in the immigration tensa barrier zig zag, then again at the baggage carousel, then the bus to the car park. Having a negative test 72 hours before travel is not going to negate any of those interactions.