If you look at this fluoridation map of England and Wales you will see that local health authority fluoridation schemes are most concentrated in the Midlands according to the 2011 data. The biggest area for this is roughly from the north of the Cotwolds as far as Stoke on Trent. Further to the north-east, another major fluoridation area starts in the north-eastern Midlands above Leicester and extends as far north as Kingston upon Hull. Northern England up to the Scottish border also has a substantial amount of fluoridated water, mainly in the north-east. Some areas, such as parts of East Anglia, have no fluoridation schemes but are naturally quite high in fluoride.
At present there are no areas of fluoridated water in Scotland, Wales or Northern Ireland - Scotland has made plans in the past to introduce fluoridation but it hasn't happened yet.
You can find more information from the British Fluoridation Society.