IIRC there's some kind of fairly old UK law that says that pubs have to provide drinking water to 'bona fide travellers'. And licensed establishments generally in at least Westminster and Camden have to provide tap water.

It's a v interesting book! I'd expect your local library to have a copy - certainly the City of Sydney library system has at least one, viz, the one I borrowed, which can be requested for $1 :) (dunno how far out CoS stretches in terms of libraries.)