ok ive done some research for you and here you go. btw if i change my exhaust your welcome to the old one.
JAPAN:
Mufflers when tested for noise output will be tested at a 45 degree angle at a 50 cm distance.
For vehicles 10 years and younger the max level allowed is 96 dB (decibels). For vehicles over 10 years of age the max level allowed is 103 dB.
UK:
Basically there is no legislation in the UK that sets a max noise limit for production cars. There is for motorbikes, and for kit cars and specials, but not for production cars.
It is an offence under C&U (I think it is) to modify the exhaust so that it is noisier than "standard". It is not an offence for the exhaust to be noisy if that is how the car was made.
References: wikipedia and pistonheads