You can get the Cupra Spoiler parts yourself, its just not cheap as you'll need:
Spoiler itself.
A mounting piece of plastic.
The black plastic trim that sits underneath (that from memory screws into the piece above).
Rear washer pipe extention
Rear Brake lights.
There is another option for the ST, but requires deeper pockets. You can buy the Cupra R St Spoiler, but that's more like £750.