About 10% of the water supply is fluoridated, ie has specially added fluoride. A small number of areas also have naturally high levels of fluoride; these are mostly in the Midlands. Fluoridation schemes operate only in England; there are none in Wales, Scotland or Northern Ireland although it is under discussion in Scotland, and there have been small schemes in Northern Ireland in the past.
In England, five water companies fluoridate their water supply. These companies are in Northumbria, South Staffordshire, East Anglia, the Midlands (Severn Trent) and the North east (United Water). There should be a fluoridation scheme introduced in the Southampton (Hampshire) area later.
Some fluoridation schemes have been paused or halted, either because of lack of local support or, more often, for financial reasons.
A good source of information is the British Fluoridation Society website, where you can also find a fluoridation map.