Britain is taking a long Christmas holiday from the rigors of recession and Thatcherism. Rush-hour traffic has already thinned to a tickle, factories are empty and offices dark, while department stores and airport waiting rooms are teeming.

For many, perhaps a majority of Britons, Christmas this year began last weekend and will continue until the Monday after New Year's Day. Faced with universally gloomy forecasts that the new year will be even worse than 1980 for the battered British economy, they have extended the country's traditionally leisurely Christmas hiatus into a full two-week escape from it all.

Many factories, including those in the ailing auto, steel and textile industries, have shut their doors until Jan. 5. Most government and many private offices will be operating with only skeleton staffs after closing completely for at least four days beginning Christmas Day. Newspapers won't be published for three days, and even public transportation will come to a standstill in most of the country for two days.

Families with enough money or credit to spare have jammed stores in a last-fling buying binge and crowded onto trains and planes for holiday vacations. Stay-at-homes already are snuggled comfortably into a two-week cocoon of morning-to-midnight television specials and movies.

The calendar and the economy are the principal reasons behind Britain's longest-ever Christmas shutdown. Britons celebrate as holidays both Christmas and the day after, called Boxing Day and traditionally reserved for exchanging presents and visits to family and friends. Since they fall on Thursday and Friday this year, a minimum four-day workless weekend was assured.

Because if its financial difficulties, the British national railroad made the unprecedented decision to cancel all train services on both Christmas and Boxing Day. And for the first time in its history, the London bus and subway system, the world's largest, will not operate on Christmas day.

Many factories decided to close completely for 10 days to two weeks because of lack of orders, the expense of heating and lighting during the year's darkest days, and traditionally high absenteeism during the holiday period.

Most workers are claiming vacation days saved this year or borrowed from the next during the two-week shutdown.But some are accepting less pay in the hope it might help avoid layoffs or permanent losses of jobs as business failures and the unemployment rate continue to increase rapidly.

But perhaps because the outlook is so dismal, Christmas shoppers have set new holiday season sales records all over the country. Only car sales have been unaffected by the sudden buying binge that retailers attribute to a desire to enjoy a bountiful Christmas before harder times ahead. They note a much higher proportion of purchases than usual being bought with credit cards, and do not expect the brisk business to continue after Christmas.

Tens of thousands of Britons, most of them from more affluent southeast corner of the country around London, also have been flocking to France for one-day shopping trips on English Channel ferries. They are stocking up in the markets of French Channel port towns on wines, cheeses, confectionary, clothes and kitchen goods.

Their money goes much further in Calais, Boulogne or Dieppe than in London because of the high exchange value of the British pound sterling and lower French taxes on wine and spirits. The day-tripping shoppers also can buy duty-free liquor on the ferries while crossing the channel. Their savings can easily cover the special winter day-trip fare of about $12 to $25.

More ambitious British travelers are spending Christmas on the continent or in warmer climes. An estimated 75,000 Britons have bought package tours abroad for the holiday season, and thousands more have been taking advantage of the lowest-ever air fares across the Atlantic.

Culture buffs staying in Britain can choose among traditional holiday pantomime performances and "Nutcracker" and "Cinderella" ballets, plus newly released blockbuster films like "Flash Gordon." The British also have a holiday mania for quizzes about everything from current events and sports to show business trivia and the royal family. The proliferate this week in newspapers, magazines and prime time television.

By far the most popular holiday pursuit is watching television. The entertainment hours of Britain's two public and one commercial newtwork have been extended during the two holiday weeks and filled with holiday specials, and popular movies, many being shown for the first time on television here.

Besides "The Godfather," "Nashville" and "Midnight Cowboy," there are eight films starring Fred Astaire, seven about Lassie and five featuring Walter Matthau. There is also the perennial showing of "The Wizard of Oz."

Britons are treated to a Christmas day television address by the queen and a holiday message from Prime Minister Margaret Thatcher. She again repeated this week her belief in her government's austere economic strategy despite the forecasts that everything will be worse except the inflation rate next year.

"It will be another hard year," Thatcher said of 1981. But she insisted that if her government sticks to its strategy "there is real hope that a year from now things will be looking distinctly brighter."