Because you have a decat lol. Simple. There isnt any other way of getting around it, the data from the emission check will see that, plus the guy who looks under your car will see it too lol.
VOSA are just money grabbing c**ts. I dont see why all the emission crap is enforced if im honest. We are going to die at the end of the day anyway. Plus look at all the third world countries polluting the world. We might aswell just join in and have fun while we are at it lol.
MOT's are bad enough on their own as it is in regards to car safety. I say that in the way that its not really strict enough. Once you have done MOT's yourself, you think f**k me, that car isnt safe and personally I wouldnt have let it passed - but according to the book, it met the minimum requirements to pass. Its a joke. MOT's are just there to say 'yep, the car on this day at this time met the minimum requirements to make it road safe'. But when it comes to servicing and other stuff, its totally different normally, the stuff that passed on the mot would be like 'hmm well that aint very good, best tell the owner to get that changed'. Cars eh lol. I dislike being a mechanic sometimes

On the subject of MOT's. It wouldnt supprise me if over the next few years they changed the guidelines again to make them more strict and maybe in the future make it so you have to have 2 a year.