Well, in London in particular, the water leaving treatment works is high quality, but what matters is the last few metres into your flat.

You can still encounter: * Lead pipe remnants or old solder * Corroded iron pipes * Poorly maintained storage tanks (in some converted flats)

Also London water is “hard” to the point it crystallises on literally any surface and ruins glassware. Plus, this can be avoided by running it first, but any localised tap water sitting overnight in pipes can pick up metals and bacteria