I agree entirely with mk375 except the part of a “guaranteed fail”. *
I also have a non catted manifold, which passes the MOT.
(BBR are certainly trying to have their cake and eat it. They are not transparent in the sales blog etc).
*If it was, then at least one of our modified cars would have been failed by now. I have researched and found NONE that have a catless manifold have been failed. Quite a few people state it should (I don’t) unless someone can prove to me with evidence that it should.
I will state again, are all the Testers turning a blind eye or incompetent? No of course not.
Likewise, it has been confirmed that a Tester is NOT allowed to use prior knowledge when testing a car.
You have to test it on what you can see and what the emissions results are.
I am still convinced if the car passes the emissions test then that’s what all the legislation was written for.
Likewise, when a central cat fails and is replaced with an aftermarket one, then are people saying that “Technically” that should also fail?
Quite a few people have replaced the central cat and not been failed because it is not the original Mazda one.
You could say the same about aftermarket suspension, brakes or whatever…
As stated above numerous threads on here and have a read of this one regarding the debate.
A can or worms indeed.
Good luck with your mod no matter which route you take. 
