These are the limits concerning aero...
The extent of aero that can be fitted to your car tends to increase as you move up the classing structure. Here is a basic guide:
Clubman Class: OE-style aero only
Pocket Rocket and Club Classes: Rear wings should not extend above the roof or from the rear of the car by more than 150mm and should be no wider than the car itself. Additional front aero should not extend more than 150mm from the car. Flat floors are not permitted.
Club Pro and Pro Classes: As per Club Classes, expect flat floors are permitted.
Pro Extreme: Unlimited.