I think it is also important to point out that there is a difference between voiding a warranty and denying a warranty claim. Most of the discussion concerning modifications deals with the denial of a claim not the rendering of the entire warranty null and void. Here is what our (US) warranty states:
BMW will not accept liability for any parts or accessories not approved by BMW.
This warranty shall be null and void if the vehicle identification number has been altered or cannot be read, if the odometer has been replaced or altered and the true mileage cannot be determined, if the vehicle has been declared a total loss or sold for salvage purposes, or if the vehicle has been used in any competitive event.