Not only is there a royal wedding in the offing, with all the anticipatory paint-up and spruce-up activity, but the sun shone for four days running last week on the Wimbledon windup, the Henley Regatta and the cricket test match between England and Australia at Lords.

The House of Commons was in such a schoolboy mood during the question period the other afternoon that the Speaker complained he could not hear the prime minister's replies, and tolerantly shushed his charges with the smiling comment, "I don't know what's got into you today."

But appearance are deceiving. The bright sun, fresh paint and high spirits cloak a grim economic situation and a strained, tense political passage. Unemployment reached its postwar high last month and is climbing toward the 3 million mark -- a rate of joblessness 50 percent higher than that in the United States. The inflation rate, where the government of Margaret Thatcher has had its greatest sucess, is nudging upward again -- as are local levies and consumer taxes.

Producation is well below the levels two years ago when Thatcher came to power, and there are complaints even from the Tory benches in Parliament that the capital she was counting on to modernize British industry is being drawn to the United States by high interest rates.

The cutbacks in social-program spending, which so far have been felt mainly by the poor, are beginning to hit the middle class as well. The government announced last week it would eliminate scholarships for 12,000 university students, thereby jeopardizing some 3,000 teaching jobs in the next three years.

All this is souring the mood among the Conservatives, and there are increasingly harsh and public complaints that Thatcher's "doctrinaire" monetarist policies are paving the path to political repudiation. Former Tory prime minister Edward Heath told a conference of British business Leaders last week that his successor's "incomprehensible policies" were not only hurting the economy but breeding bitter social unrest.

"Of course if you have half a million young people hanging around on the streets all day, you will have a major increase in juvenile crime," Heath said. "It is inevitable." The nights of rioting in Brixton and Liverpool that came immediately after his speech provided tragic confirmation.

Heath himself has no great following, but his commnts are reflected and amplified in the private remarks of younger Tory politicians. They voice a double concern: that Thatcher's rigid economic policies may be accompanied by ever-more-repressive social policies, and that both will be repudiated by the electorate in the fall of 1983 or the spring of 1984, when her present mandate expires.

There are widespread predictions of a cabinet shuffle this summer or fall, with Sir Geoffrey Howe leaving the chancellor of the exchequer post where he has been defending Thatcher's economies. But there is no sign the change of personnel would signal a change of policy.

Meantime, the fragmented opposition is preoccupied with its own problems. Labor is facing a late September showdown battle for the deputy leadership between its left-wing ideological leader, Tony Benn, and his right-wing antagonist, Denis Healey. The tide in Labor is to the left, but Benn (who has worked himself into a nervous disorder that is, at least temporarily, confining him to his home) is so prickly a figure that he may not prevail.

But even Healey concedes that a victory this fall would be but the first step in a long and painful effort to uproot far-left control of the constituency Labor parties, which are more impoverished and politically impotent than they have been for years.

The drama that has caused the press and public imagination is the effort of a new center coalition to be born. The Social Democratic Party launched in March by four ex-Labor ministers disgusted by leftist domination of their old party, and supported now by 15 members of Parliament, is attempting to work out a political deal with the Liberal Party, which has been struggling to represent moderate opinion.

A poll last week showed that in a mock general election, a Liberal-SDP alliance would win 39 percent of the vote, compared with Labor's 32 percent and the Conservatives' 27 percent. An article by Ivor Crewe in Public Opinion magazine estimates that 39 percent would yield enough seats in Parliament for the center coalition to form a majority government.

But the real world has complications not reflected in the poll. The Liberals and SDP are uneasy partners, experimenting with a system where they will cross-endorse each other's candidates in alternate by-elections. It is off to a rocky start.

Roy Jenkins, the former deputy leader of the Labor Party, is standing for the SDP with Liberal endorsement in the first by-election later this month. But it is being fought in Warrington, a Labor stronghold, where he is given little chance of winning.

The next by-election comes up in a more promising Tory seat, but there the candidate will apparently be a Liberal who has been beaten previously in the district. Meantime, Shirley Williams, the former Labor education minister who shows up in the polls as SEP's most popular figure, is waiting for her spot -- and drawing criticism for taking no risks in support of her convictions.

The SDP -- whose leaders also include former Labor foreign secretary David Owen -- and the Liberals represent a source of capable, moderate leadership.Their shared belief in a constitutional change to proportional representation in Parliament is supported by many outside their ranks, as a practical step to reduce the polarization of British politics and facilitate more moderate economic and social policies.

But that is a distant hope. So for now, the British are taking what consolation they can find in the weather -- and, of course, the wedding.