Oddly my current daily, an Impreza WRX is ULEZ compliant as it's a petrol Euro4, my Truck and the wifes van which are both 2 years newer are not because the are diesel. Diesels need to be Euro6 compliant, this will be for Nitrogen Dioxides of which the petrol is much lower. The Diesel needs an adblue Euro6 engine to drop to this level.
The only A2's that will be compliant will be the petrols as they were Euro4 compliant, the diesels are also Euro4 but this is too low.
Carl.