In my copy of the WRAS Water Regulations Guide it menitions the "water undertaker" a lot but does not seem to identify who this is. You have to inform the "water undertaker" for example if you install something from a list of items that includes, for example, a water softener.
i don't think the "water undertaker" is the local building control becuase things liek installing a combi boiler does not have to be informed to the undertaker and I think it does to BC.
Robert