the cupra runs 235/35/19 I'm not sure if you will get any wider in there if you're looking for more grip though better tyres will be cheaper then new wheels.
The Cupra also has little plastic wings on each wheel arch to make the car legal, as the 235 size has the wheel just outside the body line without them.
Only applies for new car productions to meet EU rules wont affect the MOT.
Thought any vehicle that had it's wheels protruding from the bodywork was illegal and you can get a ticket for it, never mind failing MOT.
I´ve got spacers (15mm in front and 20mm in the back) on my Leon FR with the factory 18x7,5 inch rims (225/40-18 tyres) and it´s still not protruding beyond the wings, you can´t see the thread when looking from above!