The water quality is important because it affects the taste, colour and fragrance of tea. Because the tap water in the UK is hard water, so I would recommend to use soft mineral water. The amount of CaCO3 of water decides water hardness.
Japanese tea association recommends to use water which CaCO3 level is between 30-80mg/l.
The formula to calculate CaCO3 is below.
2.5 x Calcium/l + 4.1 xMagnesium/l
In Japan, tap water is soft so not many Japanese realise how the water hardness effects the taste of tea.
I would recommend to use soft mineral water, especially when you brew premium Sencha, Gyokuro, Matcha because for these tea Umami takes very important part of their taste.
