I'm afraid you can't race it. It's a special car, meaning it requires more data from the PS2 processor to er... process it. The processor can only handle so much data, so whenever the Caterham is used, no other cars can be. The same hap`pens with other cars like the Chrysler Prowler, Auto Union '37 and many others.
There is a list (I think in the GT4 FAQ) with all the Special Cars that can't be raced.
EDIT: Here's the list of cars that can't be raced in Simulation Mode
Auto Union V16 Type-C Streamliner 1937
Benz Patent Motor Wagen 1886
Caterham Fireblade 2002
Chrysler Prowler 2002
Daimler Motor Carriage 1886
Ford Model T Tourer 1915
Jay Leno Tank Car 2003
Nike One 2022
Toyota Motor Triathlon Race Car 2004
Oh, and it's Catherham Fire
blade, not Fire
storm 