Im not sure about on the Boxster but generally on most modern vehicles you have up-stream and down-stream oxygen sensors.
Meaning one sensor reads before the cat converter and another reads after it.
If you remove the cat, both sensors will get the same reading which can sometimes flag an issue to the computer that the converter is faulty which will lead to not passing MOT
Sometimes there are ways around this but likely none of them would be deemed legal by your local MOT inspector.
-Matt
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