Tap water in london isn't particularly good, when I stayed with my brother in manchester the tap water was pretty decent but he kept brita filtered water in the fridge which tasted slightly better, but realistically I was just more used to the taste. I either drink bottled, and I prefer sparkling as it tastes more refreshing to me, or filtered water.
Tap water has gunk added to it and goes through miles of piping of various quality, the point of bottled water fresh from a naturally filtered source is its cleaner and tastes better, which it does, the very cheapest/worst water is just tap water bottled up and resold, but then its probably filtered anyway so saves you the hassle of having a ackward shaped brita filter in the fridge with a fairly small capacity and remembering to keep the thing full all the time so meh.
THen theres the zombie apocolypse, one zombie falls into the resevoir and you don't want to go near the stuff.
EDIT:- lol, bottled water is likely to be dirtier as it sits on shelves for ages, in their open dust contaminated bottles?