It is quite simple.
A catback exhaust only replaces the exhaust behind the Catalytic converter,
It will only make the car louder and doesn't interfere with the primary exhaust flow through the catalytic converter and all the sensors around it.
When you change the exhaust for one with a bigger downpipe (which nearly always include a 200 cells race metallic sports catalytic converter) the flow of the exhaust gas is directly influenced.
This means when you f.i. have a turbo
problem Seat UK can blame it to the bigger downpipe and 4sure will.
Only
help is your dealer who could keep you out of the wind not telling Seat UK.