I think you might be thinking of the cold water tank in the attic that was historically used to feed decent cold water pressure to the upstairs taps only. This

I think you might be thinking of the cold water tank in the attic that was historically used to feed decent cold water pressure to the upstairs taps only. This is why I was told to never drink out of the taps upstairs as a kid. Think these up much rarer these days. Hot water is always stored in an insulated hotwater cylinder in the UK, not open.