FOR THOSE who agree with Ratty that "there is nothing--absolutely nothing--half so much worth doing as simply messing about in boats," the canals of England and Wales offer a splendid holiday.

At the dreamlike speed of four miles an hour, a limit set by the British Waterways Board, the canalside meadows and woodlands are sure to evoke memories of ''The Wind in the Willows,'' Kenneth Grahame's magical account of the adventures of Ratty and his riverbank friends. You may even be lucky enough to catch a glimpse of Ratty himself, that knowledgeable denizen of the waterways who is not, taxonomically speaking, a rat at all, but a water vole.

The idea of hiring a canal boat appealed to us, two denizens of a small town on the Connecticut River, because we wanted an active outdoor vacation but one without strenuous physical exertion. And since we did not want to be part of an organized group, we formed our own party by inviting a friend to join us and share expenses. Three women whose average age is 60, we proved to be a capable crew for our 40-foot narrowboat "Phyllis," one of the Sundowner Boat Company's fleet based at the village of Weedon, Northamptonshire, on the Grand Union Canal in the English Midlands.

Britain's network of artificial waterways dates from the mid-18th century when the first canals and towpaths were built for horse-drawn barges that connected the new mining and manufacturing centers with already navigable rivers. Some 2,000 miles of waterways were created before competition from the more efficient railways forced most canal companies out of business. The remaining commercial carriers were for the most part individually owned and were often operated by whole families whose only home was their boat. These boat-people developed the colorful styles and designs that characterize the modern narrowboats of today's thriving tourist industry.

From the moment we boarded "Phyllis," we felt comfortably in control of our holiday. When we had unpacked our belongings and put away the groceries we had bought that morning, Ian, Sundowner's wharf manager at Weedon, gave us a quick lesson in handling the boat. After explaining such simple but basic details as the heating system and how to start and operate the diesel engine, he took us on a trial run a quarter of a mile or so along the canal.

All three of us had had some experience in handling small outboard motorboats, but getting used to a narrowboat's length and weight took practice. We learned that the bow responded slowly to the tiller and that one has almost no steering control in reverse. For these reasons, and because the canals are narrow and subject to silting, it is possible to turn around only at what are called "winding holes"--places where the canal widens to two or three times its usual width. Like other points of interest, practical or historical, the locations of winding holes are shown on mile-by-mile maps printed in pocket-sized guidebooks available from British Tourist Authority offices in Chicago, Dallas, Los Angeles and New York, and in canalside shops in Britain.

Our first winding hole was conveniently located at the end of our trial run with Ian. After guiding us through the maneuvers required to bring the bow full circle, Ian hopped off to the towpath and waved a cheery good-bye as we chugged away on our own.

Our course was north between fields of sheep with their new lambs. Moorhens paddling at the sides of the canal scuttled up the banks at our approach. One refused to leave its nest and was rocked in our wake as we passed within six feet of the branch on which the nest was cunningly constructed--a basket of woven grasses and twigs that rode gently up and down on the water. Further along a mallard ushered her 12 ducklings into the cover of marsh grass. An occasional gray heron, similar to our great blue herons, flapped lazily over the canal.

The generally flat farmlands soon gave way to rolling hills. Our guidebook (No. 2 in Nicholson's Waterways Guide series) showed that we would reach our first locks at Whilton, about five miles north of Weedon. The book also gave a clear explanation, with diagrams, of how locks work, and we felt confident we could handle them.

We tied up along the towpath below the first lock and walked ahead to open the gates. Most of the locks on the Grand Union are double locks, meaning they are wide enough for two boats to go through side by side. To conserve water, the Waterways Board urges boaters to double up in these locks whenever possible. Just as we were opening the lower gates, an exercise accomplished by pushing one's backside against a heavy balance beam, another narrowboat rounded the bend, and we joined forces.

"Butterweed," about 10 feet longer than "Phyllis," had a crew of six, two of them young teen-agers eager to work the locks. During the next two and a half hours we struck up a cordial relationship as our boats negotiated seven successive locks together, climbing 63 feet in the process.

The ascent accomplished, we declined an invitation to join the crew of "Butterweed" for supper at a canalside pub because we already had a roasting chicken in our oven. But we were to see them again on "the cut," as those who frequent the canals customarily call them. They were just the first of many friends we made over the next few days.

With a few exceptions, described in the guidebooks, narrowboats may be moored for the night almost anywhere along the canals. We chose a spot at the edge of a pasture where several cows watched disinterestedly as we hammered stakes into the ground and secured lines from our bow and stern. Then we opened a bottle of white wine that had been chilling in the refrigerator and sat down to our roast chicken, rice and a tossed salad.

Good food and drink, for those who care, are essential to a happy vacation. To cut costs, and because cooking in a snug, well-equipped galley can be satisfying, we ate most meals aboard. But on several evenings we moored near pubs (indicated on our maps by a beer mug symbol) and enjoyed traditional pub fare like shepherd's pie or fish and chips with a pint of bitter or lager.

Under Water Board rules, boats may not travel between sunset and sunrise. The short nights of spring and summer in England (we were at the same latitude as Labrador) allow time to cover 30 miles or more a day if you keep moving. But much of the joy of canal boating is in not having to get anywhere.

You can stop to explore an ancient churchyard, visit a village to shop for food and souvenirs, or just slow down to watch sheep dogs at work, to study and photograph the gracefully arched bridges that span the canal or the intricately woven hedgerows along the towpath. Bird watchers are rewarded with the chance to see skylarks, chaffinches, magpies, wagtails and less common species.. With such diversions we averaged only 15 miles a day.

Yet the scenic variety of our one-week cruise was greater than we had anticipated. For a few miles we paralleled a rail line where sleek intercity trains whizzed by at 110 miles an hour. At times the meandering canal bordered the back yards of private homes with exquisite, small gardens that often extended down toward the water and could be seen only by boaters. We traversed an eerie, mile-long tunnel near Braunston. Northeast of Rugby, the canal rode an embankment with forests on either side and crossed an acqueduct high above the River Avon. Traveling barely a hundred miles, we nevertheless gained a strong sense of the English countryside.

A week or two on the canals isn't for everyone. But for those who also agree with Ratty that it doesn't really matter "whether you arrive at your destination or whether you reach somewhere else," it is a beautiful way of touring England.