In the first instance we would advise that you approach your neighbour directly in an attempt to resolve the situation. They may not be aware that the volume level is so high it can be heard by others. If you have already tried this you should contact the Environmental Health Department at your local authority - most departments have 24-hour cover so can be contacted at any time.
For long-term problems, you may be asked by your local authority to keep a diary of information about the noise, such as when it occurs, how loud it is etc., and in some cases you may be provided with some equipment to record the noise. These two steps are designed to gather evidence.
If the council then decides that someone is causing a statutory noise nuisance, they must issue a 'noise abatement' order (i.e. a chance for them to turn the noise down), which if broken can lead to a fine, although it is likely they will give your neighbour the opportunity to turn the TV / music down without further action. The whole process can take some time to complete.
The council may also issue warning notices in response to complaints about noise above permitted levels from 11pm to 7am. These warning notices can be used by councils for noise that’s not a statutory nuisance.
See the websites in Related Information to find your local authority and to report noise pollution.