London — In a few short sentences, Jean S., a pleasant, middle-aged woman standing in her Surrey garden, summed up why Prime Minister Margaret Thatcher is expected to win what analysts are calling the most significant election in Britain since 1945:
''All this unemployment is really not just her fault,'' said Jean, mother of two now-adult children. ''It's all over the world, isn't it? Labour didn't do the job at the end of the 1970s. . . . She's the only politician who says what she thinks and then does it.
''One term in office is too short for what this country needs. We're in a mess and she deserves another term to keep trying to fix it up. . . .
''And these nuclear missiles (she meant the cruise as well as the independent British deterrent). I don't like them any more than anyone else does, and I wish we didn't have to have them, but what good would giving them up do? It would only encourage the Russians and make Britain's voice weaker in the world.''
This is the voice of pro-Thatcher, middle-class Britain, and specifically of southern England. Both are expected to give Mrs. Thatcher a handsome victory June 9.
The woman, and other voters contacted by this newspaper, spoke as Britain faces a watershed in its postwar history.
In 1945 voters had to decide: more Winston Churchill and his wartime style of leadership? Or a new peacetime era?
They switched Britain to a new track by choosing no-nonsense Clement Attlee and a Labour Cabinet that set in motion a postwar society of enhanced government powers, from welfare state to nationalized industries. That state has largely continued intact despite periods of opposition Tory (Conservative) rule.
Now, on June 9, voters must make a choice equally momentous in its own way: another term of Mrs. Thatcher and her rightist, monetarist, Reaganesque encouragement of industry self-reliance at a time of stiff international competition? Or a return to concerted state spending, state borrowing, and state-led ''reflation'' to bring down unemployment?
The economy seems to be the dominant issue here, though the United States has more interest in British foreign policy. Washington prefers a Tory victory that would retain US bases and medium-range cruise missiles as well as Britain's own nuclear deterrent, and membership in the European Community.
Labour has pledged to remove US bases, to ban cruise missiles, and to take Britain out of the EC in four to five years. These pledges may prove exceedingly difficult to carry out in office, and they have split Labour right from Labour left. But US officials say they would jar and disturb NATO at a time when the alliance ought to be united against the Soviet Union.
The election is about national mood and self-confidence. Is Thatcherite self-reliance the road to recovery - or is it Labour's answer as laid out in its election manifesto released Monday?
Labour would pump (STR)11 billion ($16.5 billion) into the economy in its first year and concentrate on creating jobs through building houses, repairing roads, and other public works.
In 1979, British industry - and national mood - were in decline. Mrs. Thatcher defeated the Labour government of James Callaghan with the force of a fervent, almost missionary campaign. She exuded confidence and certainty. She promised lower inflation, lower taxes, and more individual opportunity.
With inflation down but unemployment way up, she asks Britain for a second five-year term to complete the job. The prospect is that Labour cannot prevent her, even though it is expected to do better than its currently dismal standing in the polls would indicate.
The key swing-vote area that went to her in 1979 was the skilled blue-collar west Midlands, centered on Birmingham. Predominantly working-class northern England, Scotland, and Wales voted Labour, as they traditionally do, while prosperous southern England was and remains solidly Tory.
The remarkable aspect of her strength is the way it has endured, and even grown, through four years of rising unemployment and factory shutdowns in the west Midlands and elsewhere.
''For the six-sevenths of the population which have jobs, life isn't so bad at all,'' explains a source close to Mrs. Thatcher. ''It's for the other (one-seventh) that life is grim indeed.''
The Labour Party struggles in vain to close the Tory lead. The party is deeply divided into three segments: a young, often college-educated, grass-roots far left; an older, more moderate left centered on party leader Michael Foot; and the center left, symbolized by former Chancellor of the Exchequer Denis Healey.
Mr. Foot himself presents a poor television image, more of an absent-minded professor than a dynamic national leader. Mr. Healey doesn't support much of the manifesto's unilateralist foreign policy and has to endure constant questioning on the issue.
Meanwhile, the alliance of the Social Democratic Party (SDP) and the Liberals is lagging. It is deeply worried that Labour will in fact recover some of its traditional ground closer to election day.
Talk of a ''hung Parliament'' in which the alliance might hold the balance of power was rife late last year but is less audible today. The Tories have pushed through new electoral boundaries that benefit them by creating more suburban seats.
According to one widely quoted analysis by Prof. Ivor Crewe of Essex University, the Tory majority in June could be as much as 136 seats of a total 635. Recent opinion polls give Mrs. Thatcher the astonishing level of support of about half the electorate. She is about 20 full points ahead of Labour, with the SDP-Liberal alliance trailing.
Said a Surbiton, Surrey, printer, in a conversation at a schoolboy soccer tournament: ''Labour says it will put 21/2 million back to work in five years, but where will it get the money?'' On the other side of the coin, the wife of a lithograph printer in a more working-class area of Surrey had just paid (STR)2 as a membership fee to the country's leading peace group, the Campaign for Nuclear Disarmament.
''Britain is too small an island to have nuclear weapons,'' she said. ''They're only a target in a war.''
What did she think of the Soviet Union?
''It's just a blank to me,'' she said. ''Just dark and mysterious. All I know is that the nuclear weapons shouldn't be here.''
Jean in Surrey did have two worries.
The first was that many Tory supporters might stay home in the belief that Mrs. Thatcher was sure to win anyway. The second was that many people might be tempted to vote for the SDP-Liberal alliance in the same belief that the result was a foregone conclusion.
Tory party chairman Cecil Parkinson recognizes both dangers. They could, he says, ''put [Michael] Foot into Downing Street with the support of less than a third of the electors.''
In May 1979, the Tories won 339 seats, Labour 268, Liberals 11, and others 17 . When Mrs. Thatcher called this election, she held 334, Labour had dropped to 239, the new Social Democratic Party had reached 29, and the Liberals were up to 13..