ive been sent to DECC once unfortunately and went to Granville.
Car has to be put back to standard completely ( I got away with powerFC, injectors on standard fuel rail & tomei 260/260 9.15mm cams) but everything else had to be put back to standard.
Your car will not past even if your car is 91db, I had a standard r34 gt-r cat and exhaust and it still failed.
I had to have metal plates put in my exhaust and drilled little holes through it. Personally to pass i reckon the best way is go to the exhaust shop and get them to make you a whole new exhaust system, a small one, one or two cats and one or two midmufflers, its almost impossible to get your car to 90db in my experience. But this is definitely what i would do in the future, you can keep your modifications on if your car is engineered but be sure they'll send you to do an emissions test afterwards!