Fluoride occurs naturally at very low levels in drinking water, the amount depends on the type of the rock near the source of your water supply. 

Water companies are sometimes asked to add fluoride to improve dental health.  The amount of fluoride allowed in drinking water is much lower than found in products such as toothpaste.  The UK regulatory limit for fluoride in drinking water is 1.5 milligram per litre (mg/l). Water companies that treat the water will only add fluoride to water supplies when requested to do so by the local Health Authority. On average the level is only raised to 1.0 milligram per litre (mg/l). 

Public Health England and the local health authority pay for the fluoridation and equipment. 

The final decision on adding fluoride sits with the local authorities and the Secretary of State for Health. Water companies, if approached, give technical advice and if asked to do so, will implement a fluoridation scheme in line with the relevant Act and Regulations. The decision to fluoridate (or not fluoridate) is not made by the Water companies.