Tap water for mine, they seem to prefer it that way! The reason cats often dislike tap water because of the chlorine, but some cats will tolerate it (the chlorine dissapates on aeration of the water or leaving it to sit). i've got a Catit water fountain which recommends that tapwater is used. Filtering helps to remove some of the impurities so go for it as it'll mean the water doesn't need to hang around so long!
British tap water is very well regulated on the whole. I looked into it when I was having a problem with nitrate levels in my fish tank, to eliminate it as a source. It can contain no more than 10ppm, so the comanies mix it to keep levels right.
Bottled water is highly variable due to the fact that it often originates from one source and can't be mixed. I drink fizzy water as I find it refreshing with fruit juice but I don't believe it's any better than tap!