As they are hand made, they probably just build up a new chassis where needed from the original jigs.

There will have to be a certain proportion of parts salvageable from the damaged car to retain its ID.

Although in theory Cat A is supposed to be a car thats burnt out, or has no salvagable parts it often includes cars where a serious injury or fatal has occurred and its covered in blood/brains etc. Eg itsquite possible to run someone over and kill them or a driver dies at slow speed because they are too mongaloid and didnt put a seatbelt on, yet the car has relatively minor damage.

A lot of cars get mis categorised too. There is some evidence to suggest this is the case with older cars to get them off the road by cat B'ing cars that are perfectly repairable.

Also flood damaged cars are mostly cat B when in reality if stripped to a bare shell can with proper treatment be saved.

Only really worth it with rare cars or a car with a special history, hence why aston offer the service.

You wont find ford doing the same for your mondeo no matter how much you pay them!!

Oh and looking on the specific link - being a category A repair dealer is not the same as fixing cat A cars, albeit nothing stopping them as above a bit like maclaren already do.