Aftermarket backboxes are legal as long as they bear an e mark, and many have the ability for a bung to be fitted, there is no decibel test in the mot for cars, motorcyles on the otherhand are a different story, a loud deep tone is preffered to a high pitched scream endcans must have bsau markings and also be stamped for road use or not. The road traffic act states they must have the markings, but if you are handy with a dremmel then fire in and do it yourself.
Its then down to the mot tester