They are open to all. People are often working long hours in libraries and appreciate a drink. You cab sit and drink your water inside whilst using the services without feeling like you have to buy something like in a cafe, pub or restaurant. They are also accessible and most towns have one.
Also do you have any big pub chains signed up to this scheme, eg) Stonegate, Mitchell and Butlers, Nicholson, JD Whetherspoons etc. I'm sure you have probably approached them but if they are not willing to be involved then try smaller chains, independent pubs and brewery owned.