The UK food standards agency says this (and it would cover the whole of the uk):
For babies under six months old, take water from the mains tap in the kitchen and boil it. Remember to allow the water to cool before giving it to your baby.
The UK babycentre website also says this:
Tap water should be boiled for babies up to six months of age. After that it can be offered straight from the mains supply. If you wish to use a filter that is your personal choice, but do follow the filter manufacturer's instructions and keep filtered water in the fridge, as the filtering process removes some of the additives, such as chlorine, used to keep tap water fresh