Seems I failed miserably at keeping you posted. Still, better late than never... The ICO sided with WMFS. I took the matter to a tribunal, and the judges agreed with the ICO's assessment:

The ICO concluded that the withholding of the information was reasonably necessary for national security and a listing of hydrants and their locations would assist in the planning of an attack by poisoning on water supply infrastructure by identifying access points. Such an attack on the infrastructure would be in the domain of national security. While hydrants are visible a comprehensive list of the precise location of every hydrant would place in the public domain more information than is available through hydrants being visible. WMFS supplied a list of attacks and attempted attacks on water supplies.The ICO considered that such an attack was plausible.