At present legislation only provides the local authority with the power to take action against loud car stereos if the car is on private land or parked on the street. If there is a car/a group of cars that is parked up constantly on your street and the stereos are being played very loud then you need to make a complaint to your local authority.
The police also have powers to deal with cars playing loud music (whilst driving and stationary) although the officers would have to be present and hear the noise before they could take any action.
If it is a one-off occurrence then neither the police nor the local authority will be likely to take any action. If however it is a persistent problem then either contact the local authority or your local police via their 101 number.
See the website in related information to find your local authority.