I can confirm my replies were based on legal advice.
To ask Her Majesty's Government further to the Written Answers by Lord Darzi of Denham on 17 November 2008 (WA 165–66) and 17 December 2008 (WA 41) which state that water fluoridation is a service undertaken under the Water Industry Act 1991, why the Department of Health takes responsibility for it. [HL730]
Because increasing the concentration of fluoride in water to 1 part per million, where it does not occur at this level naturally, protects people from dental disease. Responsibility for undertaking consultations on water fluoridation is delegated to strategic health authorities under Section 89 of the Water Industry Act 1991.