Yeah there not allowed to deviate too far from the road car so i think they only get the basics like:
Filter
exhaust
re map?
Could be wrong tho
 
The BTCC rules :

Retain a two-litre engine capacity (developing more than 270bhp) and mass production body-shells, but limit much of the scope for modifications versus previous years. Twenty-seven specified components, including gearboxes, brakes, clutches, ECUs, wheels, fuel systems and data loggers, are now common to all cars. Teams may only purchase them from the one designated supplier to control development costs and keep the racing close.

Aerodynamic packages are intentionally limited to promote overtaking, while the use of smaller wheels (17-inch diameter) and brakes reduce grip, lengthen braking zones and again increase opportunities to pass.

The car's weight limit is 1150 kilos and use of exotic materials is prohibited for all but safety applications. The system of "success ballast", introduced to keep racing close, has proved very successful in maintaining an exciting spectacle throughout the season.
 
There's a few of them up for sale at the moment... not that I've been looking.

Might be mistaken but I don't think the shell's are registerable, thus you can't build one into a highly strung road/track toy, which would have been tempting.
On that note, saw a really cool road registered toy the other day up at Specialist cars... 993 RSR, shame I've not a spare 70k!
 
Nothing stopping you from getting a VIN from a Cat D car & 'reshelling' that car with your supercopa one :D