Hello Leon 
In terms of the vehicles you are suggesting, I personally think that that would be way too many for the Monarch to choose from. Also, the Prime Minister has used the Jaguar XJ in the past in real life, but the Monarch has never to my knowledge, so it would be unrealistic. It would also be unrealistic for them to be driving supercars.
The suggestion of adding more Royal Residences is linked here: More Royal Buildings