I live in London and the tap water is fine, tastes the same as bottled TBH. I do use bottled for convienience, to take with me and to keep by my bed (no chance of knocking water everywhere!)
However I'm from Cornwall and grew up on the cheapest bottled water available, like 17p for 2l from morrisons. The water is pretty foul in the area, though it is improving as I do try it everytime I go home.