As said above, strictly for cars there is no set Db volume that is too loud, however, i am fairly sure that in the letter of the law its not legal to fit an exhaust that is louder than the OE one, and in doing so you open yourself up to this type of pull over situatiuon.
Its upto the officer/MOT tester/Court if it goes there to decide what is acceptable.
On my old integra i had a Buddy Club Pro Spec exhaust, which even makes the K1 sound reasonably quiet compared!
Only 2 times was i pulled over, and on both occasions it was because the coppers wanted a better listen to it, and to tell me how great it sounded. Its all down to the copper and their mood tbh, you were just unlucky in this instance!