According to this:
https://www.essex.police.uk/getmedia...t-Offences.pdf
"In summary, vehicle exhausts must not be altered or replaced in a manner which increases the noise above that emitted by the type approved exhaust fitted by the manufacturer.
"In any case the sound emitted by an exhaust fitted to a standard car must not exceed 80dB. Some other vehicles, such as those with differing engine power e.g. HGVs, PCVs, motorcycles, etc. have higher limits up to 89dB. The offence also relates to vehicles being used in a manner as to cause any excessive noise which could have been avoided by the exercise of reasonable care by the driver. This may include revving engines, tyre noise and in some cases, audio equipment, etc."
I think the police need to do an audiometer test (or see you doing a burnout / donut
) to issue a ticket but the penalties for the offence are a £50 non-endorsable (i.e. no points) fixed penalty notice for ‘silencer/ exhaust system altered to increase noise’ (RC86068 ) or for ‘avoidance of excessive noise’ (RC86092).