Section 62 of The Control of Pollution Act 1974 (still) provides that a loudspeaker in a street shall not be operated between the hours of nine in the evening and eight the following morning, for any purpose, nor at any other time for the purposes of advertising any entertainment, trade or business (so political vans are OK, because politics is not entertaining!!). Any person who operates or permits the operation of a loudspeaker in such a manner is guilty of an offence. There are exemptions for the emergency services and similar.
The above provisions were modified by The Noise and Statutory Nuisances Act 1993, such that local authorities can adopt powers to grant consents for loudspeakers to be operated between nine in the evening and eight the following morning, but they first have to resolve to adopt the power, and they have to grant specific consents to each case.
Therefore if a pub plays amplified music from a loudspeaker in the street, or if a party or pop concert is held in a street after 9 pm, the operators may be guilty of an offence.
With regard to nuisance, the legislation does not state in terms that to operate a loudspeaker in a street after 9 pm, should automatically be considered a statutory nuisance; however, the fact that it is a criminal offence to do so, is a strong indication that noise nuisance is likely to occur under such circumstances