I have a friend who tells me that the non-drinking water sign you see in cafes, supermarket toilets in the UK is just to mitigate responsibility. Is this true, or what about the water could be unsafe?
Commonly in the UK, cold water may be stored in open (or roughly covered) tanks in the roofspace.
Such water, while it will have been drinkable mains water when it arrived is considered non-potable as there is no knowing what might have contaminated it while sitting in the tank.
Contrary to the other answer, it is very unlikely that grey water is coming out of a tap, though untreated water is another possibility in rural areas.
See https://www.theguardian.com/notesandqueries/query/0,5753,-2404,00.html for thoughts on why this system exists. (A classic Fawlty Towers episode sees the hotel inspectors find a dead pigeon in the hotel water tank.)
Whether this is the case in every instance is debatable.
It is a requirement of the Health and Safey Executive that water is marked as "Not Drinking Water" unless it meets the requirements to be considered drinking water.
An adequate supply of high-quality drinking water, with an upward drinking jet or suitable cups, should be provided. Water should only be provided in refillable enclosed containers where it cannot be obtained directly from a mains supply. The containers should be refilled at least daily (unless they are chilled water dispensers where the containers are returned to the supplier for refilling). Bottled water/water dispensing systems may still be provided as a secondary source of drinking water. Drinking water does not have to be marked unless there is a significant risk of people drinking non-drinking water.
Should water in a particular supermarket toilet come not directly from the mains supply but via a storage tank of some kind between the mains and the toilet it would not be considered drinking water.
The important thing to know about drinking water, in the UK and everywhere else, is that, while it leaves the water treatment works in a disinfected, safe, state, it then has to travel through many miles of pipework before being consumed. It is impossible to keep this pipework completely clean and so the water is treated with a disinfection agent such as chlorine (traditional solution, but can affect the taste of the water) or chloramine (used more often nowadays since it has less taste) that stays in the water and keeps it safe for a period of time, typically 5-7 days depending upon the strength and chemical used. The water distribution systems (the pipes under the streets) are designed such that the water will reach the consumer well under this time period and so be safe at the point of use.
In a domestic situation, where the kitchen tap, the primary source of drinking water, typically runs direct off the water mains, this is safe. In large commercial buildings however, there is a large amount of additional pipework, extra storage tanks, pumps, etc. and in order for the water to be guaranteed safe (it's not ok to be alright most of the time) all of this additional infrastructure must be periodically flushed, disinfected, and tested in order to ensure that no pathogens can be introduced into the water. This is an expensive hassle and so the simplest thing to do is to have a separate, simple, drinking water supply within the building, which does get cleaned and tested regularly, and to label all other water points as non-potable.
You probably won't get ill drinking from such water points (due to the presence of existing disinfection agents and engineering safety factors), but is it really worth a dose of the squits?
I have lived in the UK since 1974, and have never seen a sign marking tap water as unfit for human consumption. At what commercial premises, and in which town, are you specifically alleging that you have seen such a notice?
It would be highly unlikely that commercial premises in England would use (untreated) rainwater instead of the (treated) water mains, for any purpose. I know the UK has a bad reputation for it always raining here, but this is largely a myth. In reality you could not expect to meet the water demands in a commercial building, such as a restaurant or supermarket, just by putting an open water tank on the roof. Even if there is only moderate demand, you would be bound to end up sooner or later with an empty tank, and thus non-flushing toilets, especially in the (relatively dry) summer months.
The newspapers would have a field-day with such a story. But that never happens. What business wants bad publicity of that sort, especially one which sells food?
The 'non-potable' notice is often encountered in France, and is a recognised hazard of holidaying in popular areas such as Brittany or Provence; and I have even seen the notice in Paris. But British public health laws are of very long standing, and water treatment plants are fairly common: I live within two miles of one! So untreated water is rare as hen's teeth here.
The term 'non-potable' is actually French in origin, presumably because this condition is only encountered by the English when travelling, notably in France, and so was imported into our language. But even so it's not a commonly used term, and is mostly only encountered in travel guides. The word 'potable' on its own I have never seen used - not in conversation, nor in a newspaper. It's only ever used as part of 'non-potable', and (not being an English word) wouldn't be understood separately from that term. The French term has survived simply because there isn't an English equivalent: the Victorian public health laws have been in force so long that the concept of tap water being unsafe is unknown here.