The N1 is a Spoon back box, and the B pipe is just the center section, or midpipe of the exhaust. The Spoon version is a silenced center section with a larger diameter than the original CTR part, and will allow you to fit various aftermarket back boxes, as well as the original CTR back box if you want to keep it 'stealth'.
It is doubtful that you would pass an MOT with a straight through decat pipe. A sports cat will not always guarantee you will pass the MOT, but its much more likely to pass with a sports cat over a decat.