Ok, booked J-class & got a J-class (pickup from Marriott Marble Arch), they claim that no upgrades even for President cos they only get cars delivered from somewhere else on demand.
Got a bog standard Passat S 2.0TDI DSG, no options at all. Someone returned an Audi A4 before me which is also classified as a J. Guess next time I should pickup from airport or somewhere with more cars.