Claudia Pritchard: When only a Pissarro can answer your need

Ah, the National Gallery, temple of British art acquisition: the Wilton Diptych, medieval miracle of preservation and Christmas card elect; the Rokeby Venus, stripped from her native Spain; two of the nation's five covetable Vermeers; and, arguably the greatest treasure, free lavatories.

We dive into such sanctuaries for enlightenment, but they serve a more basic function too. For only those with steel bladders will have failed to notice that the great British public convenience is now a rarity. Many of the finest specimens – noble Victorian monuments to public hygiene – have been sold off by once proud local authorities only to re-emerge, supply-side, as it were, as cafés. Others have simply been shut. They cost too much to clean and maintain, harbour drug addicts, attract vandalism.

There are splendid examples – light, bright, spotless, staffed – and they charge, anything up to £1 a visit. Pay up or pop. For a family on a tight budget, that's a fiver to answer the call of nature. One London cathedral tired of sweeping away the needles has introduced a charge to weed out all but legitimate users. Understandable, but showing visitors the way to the cloakroom I file under "Come unto me all who are weary and heavy laden".

One door closes and another door closes. For years, I have been checking in to the Charing Cross Hotel for approximately three minutes and checking out again, without actually troubling the busy desk staff. Now the ladies' powder room is out of bounds to itinerants like me: only a legitimate guest's swipe entry card will secure admission.

A nation whose sense of humour is largely based on widdles and whoopsies is ill-equipped to take seriously the public health implications of the great disappearing convenience. While a good number of men clearly believe that 16 pints will be alchemically converted into solid muscle and sexual charisma, in reality, a historic building somewhere will be anointed with two gallons of corrosive piddle.

A rather alarming urinal for city centres, which rises phallically with the falling darkness, is no match for the dedicated piss artist. But older people refrain from drinking anything at all before venturing out for fear of being caught short, and so, say medics, become dangerously dehydrated.

Ryanair is to take a pound for a pee, or so it says, although we didn't believe Michael O'Leary last time he flew easily into the headlines with this penny-pinching proposal. No one uses the on-board WC capriciously, by way of in-flight entertainment. In an emergency, you dart in, you dart out. It isn't a lifestyle choice. Under the Geneva Conventions, a prisoner has a right of access to toilet facilities, which puts Mr O'Leary in the sinister mausoleum of vindictive bullies somewhere between the Marquis de Sade and Pol Pot.

What future then for the parent with a child who needs to go, for the woman whose period starts in Superdrug – or for any normal human being, in fact? She must develop a lively interest in the arts. The very excellent, free Manchester Art Gallery is just the place to freshen up when the excitement of Piccadilly Gardens kicks in. The Royal Academy loos can be reached without paying for the exhibition. And there's a wrinkle at the Royal Opera House, morning, noon and night, whereby you can get a cup of filtered water too.

National and local government alike have washed their hands of personal hygiene, but Britain's wondrous arts institutions have come to the rescue. We can repay them by bumping up their visitor numbers. They look after us, body and soul.