I own an MOT station, I also am an NT and a Scooby owner with sunstrips, small number plates, no cats and loud loud exhaust.
Firstly, you'll need a cat to pass the test, that's the bottom line. The exhaust must emit noise to the same level as a standard system on same model. There are no measuring devices and it is at the testers discretion to determine what is normal, clearly yours is not.
We are not here to piss you off, we're here to follow a set of rules laid down by the powers that be. The test is ludicrously poor in some areas and good in others. As an owner I have invested between 50 and a 100k to build to suit and install and operate a test station. If we don't test to the rules we can easily be removed from testing, bang goes the investment and there is NO way back. It's well known as stated above that stuff goes on/off for test and straight after you can put it back how it was. The test is appropriate on the day, while the car's in the bay and still on our premises (unless we're putting a car back to the way it was pre test which would have caused it to fail) and half an hour later it can be altered in so many ways (exclude corrosion from that). I see it from both sides. If your car isn't to MOT standard then either make it right then change it back, or if you are so inclined get yourself an MOT from a pub, save the time to change.
One thing - many, many cars, prob not so many Scoobs I presume because of the reasons we drive these cars, only ever get looked at once a year at test, never see a service, so see us as weeding out the dangerous stuff and not someone who's out to pick on you cause your car doesn't toe the line.
Rant over.
Oh, and all MOT stations should always come to the same result, the rule book is clear, experience is where the differences occur.