It will void the warranty on any components that might be affected by the part you have modified.
For example, if you stupidly lower the car and the cv boot splits it will not be covered as the lowering of the car has altered the angle of the drive shaft and would have put extra stress on the boot.
An exhaust could have a number of knock ons but I can't see anything major being affected. If the engine blows up it will still be covered unless RUK can prove the exhaust caused the fault.