Very difficult to get a device to be accurate enough (particularly over a very short period) to provide proof for a conviction or prosecution. There are byelaws that limit noise levels; but over a reasonable period. London is particularly strict about not increasing noise levels beyond a set amount above normal background noise.
I've had several background noise level surveys carried out in London prior to the installation of commercial cooling equipment. Often this would just limit the operating periods and sometimes attenuation was required. A very complex subject.