I hope your neighbours like the noise as much as you do.
(this next bit gets technical / legal, but is well worth knowing if you mod your car in any way that changes the noise it makes - also, see the end for my personal thoughts)
In the UK, it is an offence to fit an exhaust that is significantly louder then the original equipment. Specifically Regulation 97 of the Road Vehicles (Construction and Use) Regulations 1986 provides for the avoidance of excessive noise from motor vehicles by saying that "No motor vehicle shall be used on a road in such manner as to cause any excessive noise which could have been avoided by the exercise of reasonable care on the part of the driver".
This is an 'absolute offence', all it takes is the opinion of a Police Officer who was a witness. "If in the opinion of the prosecuting officer, your car is louder than a standard car of similar type and age, an offence has been committed."
This was originally contained in The Road Vehicles (Construction and Use) (Amendment) (No. 5) Regulations 1996
This was based on the relevant EU Directives are in paragraphs 3 and 5 of Annex I to Community Directive 92/97 or 96/20
If stopped, and people are (normally in town centres), then you will probably be issued a section 59 notice and or a PG9. Generally it is a fine and you have to remove the cause of the excess noise or risk impoundment.
This was made even more complicated by the introduction of the Environmental Protection (Noise Pollution) requirements
Police around the UK now use sound measuring equipment to British Standards and will pull people over if their exhaust noise is far above the 'type approval' range of 82-89 dB, even though there is no actual legal sound level ( dB ) limit on road vehicles. (For the novice 89dB is a large 4x4 at full throttle). At least once Police force will consider prosecution at 90dB. This might vary where you have an SVA certificate.
Finally, your neighbours have a right under the The Noise and Statutory Nuisance Act 1993
and later amendments and local bylaws to complain if you sit on the drive and rev the engine, have a loud vehicle, or use it in a way that causes a nuisance, especially at night.
I *love* the sound of good exhausts tuned to suit an engine, used appropriately and carefully matched to vehicle use and performance; I just recognise that manufacturers go to a lot of trouble to meet the regulations and I trust that all aftermarket manufacturers do, too.