Of every three British women under 20 reciting her marriage vows, one is already an expectant mother. Half of all British men tend gardens, a pastime surpassed only by watching television and dining out. Britain has lost pride of place of Japan as the world's champion consumer of newspapers but is still No. 2.

This and much more about the folkways of the 55.9 million inhabitants of the United Kingdom (where population is expected to fall by 20,000 in the 1980s, then rise by a million in the 1980s) is disclosed today in the latest edition of "Social Trends."

An indispensable almanac compiled by the Central Statistical Office, "Social Trends" has now been published for eight year. Its 264 pages of numbers and graphs measure, chart and compare Britons with each other and the world.

Sir Claus Moser, who divides his time between overseeing the Royal Opera House and the Statistical Office, estimates that more than 20 countries, including the United States, have now followed Britains lead and are publishing social indicators. These documents, drawing on the huge torrent of government statistics, attempt to reflect the well-being of a nation.

There is little in Moser's eighth edition to suggest that Britain is coming apart at the seams, a favorite lament of both visiting and home-grown journalist. Divorce and abortion have been increasing, but both have become legally easier to obtain. Abortion appears to have reached a peak three years ago and has been declining since.The divorce rate is still less than half that of the United States.

Crimes of violence are up sharply - more than four times in the last 15 years - but are still only a fraction of the number in the United States. Moreover, as Moser has observed elsewhere, crime statistics measure little except the activity of the police. Their numbers in Britain have grown from 87,100 in 1961 to 125,500 last year, and a bigger force will inevitably find work to occupy itself.

The impact of the welfare state, cushioning the worst-off citizens, is evident throughout the report. The poorest fifth, those earning less than $60 a week, receive more than half their income in social security benefits. More than half received enough to rent or buy a television, washing machine and refrigerator. The big difference between these families - typically a pensioner or a one-parent household - and those higher up the income scale is the lack of a telephone, car and central heating.

Despite Britain's relatively tough tax laws, the nation's wealth is still concentrated in few hands. The richest 1 per cent own one-sixth of all the property, shares, bonds and land; 15 years ago they owned 28 per cent.

Almost all of this shift has taken place among the richest half of the nation, however. In 1961, these people owned 82 per cent of all wealth; today, their share has slipped only three percentage points. This indicates that upwardly mobile, striving Britons are clawing wealth from those at the top but precious little filters down to those at the bottom. Britain, like other nations, is not an egalitarian society.

In one area, Britain is the undisputed champion of the industrial world. Its inflation rate since 1970 has averaged 14 per cent a year compared to the 6 per cent of West Germany and 7 per cent of the United States. A wild increase in the money supply in 1972 and 1973 accounts for Britain's significantly poorer performance. Now, after three harsh years of curbing wages, British inflation is approaching the average European level.

Britons are drinking less booze than their grandparents at the turn of the century. Despite much larger incomes, Britons now consume nine gallons of beer per person less than in 1911 and have reduced their hard liquor drinking by one-fourth. Wine drinking, however, has increased more than three times.