Those Corsa VXR's you saw were Tamiya demo cars. Vauxhall had Tamiya commissioned for their Have-A-Go cars that were used at various corporate days (I think they were based on the Tamiya M03), and were available to buy as part of an XB RTR RTR M03 retails in modelshops for around £150).
Tamiya were also commissioned to do an Astra Sportshatch and Vectra VXR in 190mm for the displays at BTCC meetings, I grabbed this pic:
Like the Astra and the Corsa, they are moulded from one-colour plastic, then all the colouring is decals.
Vauxhall might have some left over from their BTCC days, best get in touch with their marketing/merchandising department.