Fitting a separate hard water drinking tap is not required by regulation, except in exceptionally hard water areas (above 400mg/l) where the Sodium content of the treated water (increased due to the softening process) exceeds the level laid down in the Water Supply (Water Quality) Regulations 2000. Fitting a separate drinking water tap is currently recommended by the UK Dept of Health, WRAS and the UKWTA.
Source – United Kingdom Water Treatment Association – http://www.ukwta.org/technology-areas/faqs/