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 of your local authority - most 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 someone is causing a statutory noise nuisance, they must issue a 'noise abatement' order (i.e. a chance to turn the noise down), which if broken can lead to a fine of up to £5000, 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 police do not have any powers to prosecute for noise nuisance.
See the website in Related Information to find your local authority and to report noise pollution.