London — SO you think you know the British? You think that they all keep pets, right? You think that British streets are a welcome contrast to many American ones because you can walk them without fear of being mugged?
That the average Briton likes the Queen, Prince Charles and the Princess of Wales, military pageantry, soccer, cricket, Rugby football, boxing, and radio as well as television?
That this is still a nation of relaxed pace - of gardeners, walkers, and bird lovers, nature ramblers, Boy Scouts and Girl Guides, and jumble (garage) sales, summer fetes, and afternoon tea?
Well, you are right about the Queen, the pageantry, the sport, the radio, the TV, and all the small, particular, human pleasures of countryside and tea.
But you may have missed some changes elsewhere.
One example: Just over half of all adults now have no pet, according to the latest edition of the government statistical analysis, Social Trends.
It says the British now like something else better: the ''mass acquisition of desirable consumer goods.''
The British certainly like video tape recorders, partly because they can be rented easily along with television sets themselves.
Among industrial nations, only Japan now has as many households with videos.
Again, Britain is indeed a relatively low-crime country - but unemployment is high, inner cities are overcrowded, minorities are growing, and almost 50 percent of 4,000 people surveyed in a poll published in January 1984 said they felt unsafe walking alone at night.
Nor is the authority of the British bobby as unchallenged as it once was: In another January poll, 25 percent had complaints about police behavior, and 32 percent (mainly minorities) said they did not trust a policeman to tell the truth.
At the same time, crime has risen to the point where most of those questioned felt police should have wide powers. Sixty-six percent supported ''stop and search'' procedures; 62 percent approved the use of water cannons, tear gas, and plastic bullets to control riots; 61 percent agreed that mass fingerprinting was acceptable in an area where serious crimes had been reported; 66 percent accepted questioning a suspect before he had called a lawyer; and 84 percent approved telephone wire-tapping.
Other outlines have blurred over a longer period.
To the visitor, much of the countryside remains a series of John Constable paintings - patchwork-quilt meadows deep-green even in winter, long hedges, spreading oak trees, willows weeping by streams, thatched cottages, the hum and drone and buzz of summer.
Yet one-quarter of those hedgerows - some 126,000 miles of them - have been lost since 1945, along with one-half of moorland grazing, many meadows, and one-half of the ponds, according to evidence just given to an agriculture subcommittee of the House of Lords.
Conservation groups such as the Council for the Protection of Rural England blamed the growth of intensive farming, encouraged by Common Market farm prices and policies. Hedges have come down to make room for large combines. Meadows have been drained and grass replaced with lucrative cereal crops.
Meanwhile, more long-term change: The educated elite, with no empire to administer, go into different fields.
In particular, they enter the law, merchant banking, stockbroking, Whitehall (the civil service, which took one-fifth of university graduates by 1976), prestigious universities as scholars and lecturers, the BBC, perhaps the Times (whose current editor, Charles Douglas-Home, is a relative of former Conservative Prime Minister Lord Home).
Some of the elite go into industry and engineering, but many of them do not. By 1976, one-fifth of all engineering places in universities went to foreign students, and almost one-half the production engineering places in polytechnics (vocational higher education) were filled from abroad.
More change: The population as a whole is getting older, as it is in the US. The number over age 75 will increase sharply between now and the end of the century. Fewer active workers will have to support the state pension and welfare programs for more elderly.
The number of 18-year-olds is to drop by about one-third by the mid-1990s, although universities argue that the pressure to get into a university will stay high. The prestige of a degree remains very high, they point out, and the birth rate is not actually falling among those segments of the population ''most likely to seek university education.''
Other snippets of change strike at that most revered of British traditions: the hearty breakfast. This is a serious matter in a country where it has been said that the best way to eat well is to eat breakfast three times a day.
A full 8 percent of people questioned in a recent poll admitted to eating (horrors) cheese for breakfast, European-style.
The Better Bacon Bureau has felt it necessary to hit back by sending 16-page brochures extolling the traditional rasher to 11 million homes.
AND breakfast television, that hallmark of America, is catching on among children - although not so much among adults. A November 1983 poll showed 25 percent of schoolchildren watching every morning of the week, but 57 percent of adults declaring they never watch then.
But what can you expect, when the same poll shows 1 in 3 parents allowing their children a separate television in their bedroom?
Some things, however, stay the same. Well, almost the same.
Despite the telephone, the country that gave the world Jane Austen still spends a lot of time writing personal letters - 679 million in 1983, up 37 million from the year before.
The figure, compiled by an arm of the stationery industry called the Letter Writing Bureau, excludes greeting cards, junk mail, business letters, bank statements, and similar mail.
Unsurprisingly, this is a big drop from 1900 when, before the telephone era, a smaller population wrote about 1,100 million personal letters.
It is also below the figures of the 1940s and 1950s. But the national average per author per year is nonetheless 37. Those over 65 write 45 each; the 16-to-24 age group writes 36 a year, mainly love letters, and women write twice as many letters as men.
All the British still suffer pebbly, undeveloped beaches, indifferent public catering, and overcrowded roads. They like boats, wading, collecting shells, and the comfort of routine.
They like to smoke and drink, although the proportion of smokers has dropped steadily in the last two decades, to less than 40 percent of adults now.
The British spend so much time pottering about their gardens that one wonders why it has taken so long for Britain to stage one of the annual international garden festivals. Provided there is no repeat of the 1981 rioting in adjacent Toxteth, the festival is to be held between April and October this year in otherwise ailing, politically divided, short-of-cash Liverpool, on 125 acres of reclaimed dockland.
The Queen will open it, and it is hoped that 3 million people will troop through the Japanese garden, the Italian garden, and all the rest.
THE British Tourist Authority is running special garden weekends to the Liverpool area for visitors. Given decent weather, the British will flock there in droves.
This is no longer a nation of ardent churchgoers: Although about half the population (27 million) has been baptized into the Church of England, only about 1.5 million attend church regularly.
Nor is it one where women have reached public equality with men. The unemployment rate among women is double that of men, and although both the head of the royal family and the head of the government are women whose names are known worldwide, a mere handful of women sit in the House of Commons and hold high position in industry and corporations.
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