In America you hear slightly similar concerns about tank-type water heaters. We’re a bit less paranoid about them; what the Brits call “mixer taps” are typical equipment here, but you will hear people distrust drinking from the hot water heater, especially if the tank is old and unmaintained, the anode rod is shot and the tank itself is rusting.

I’m personally okay with drinking water from a water heater I know is well-maintained, and I can think of exactly three water heaters I know is well-maintained, which doesn’t include my own. I’m pretty sure the previous owners neglected it.