from NHS website:
In the event of an interruption to your supply of tap water - for example, if the water supply is interrupted, or contaminated, by a flood - you should ideally use the water from your water tank, that should be supplied by the water company. However, if this is not available, you should use bottled water to prepare a formula feed.
Make sure that the water is brought to a 'rolling' boil and is left covered, to cool, for no more than half an hour. Use cooled boiled water, or bottled water, to cool the feed once it has been made up. Alternatively, ready-to-feed liquid formula could be used instead.
If there is no electricity, or gas, to allow you to boil water, and you do not have ready-to-feed liquid formula available, bottled water (table, spring, or mineral water) can be used to prepare baby feeds without boiling it first. However, once the feed has been prepared, it should be used immediately.
In the UK, there is no legal definition for what is meant by 'spring', or 'natural' water. Some bottled water labelled as 'natural mineral water' may have high levels of sodium that are not suitable for infants. When buying bottled mineral water, check the label to make sure that it does not contain more than 200mg of sodium (Na) a litre.