Capitol wrote:
To the best of my knowledge, in a domestic installation, there is only
a suggestion to have a directly fed kitchen tap, not a requirement.

Not quite. There is a statutory requirement if you start off with
really, really hard water.

Schedule 1 of the Water Supply (Water Quality) Regulations 2000 limits
sodium at a drinking water tap to 200mg/l. So if you have really hard
water the treated water (increased due to the softening process) exceeds
the permitted level. Fitting a separate drinking water tap gets you back
on-side.

But as others have said, it's advisable for less hard water too.
--
Robin
reply to address is (meant to be) valid