EUROPE'S mood in the summer of '91 is one of curious uncertainty. On the one hand, the nations of Western Europe exude outward well-being.In London the streets are apurr with Rolls Royces and Jaguars. Though Burger King and Pizza Hut and McDonald's have staked out positions in Piccadilly and the Haymarket, afternoon tea is still served with Old World gentility at Brown's Hotel, the Devonshire cream piled lavishly on fresh, fat strawberries. In Munich, the taxis are Mercedes and BMW's. Bavaria is enjoying a boom. As throughout all of Western Europe, the hotels are clogged with Japanese businessmen and holidaying Arab families, the latter sometimes taking whole floors, whence waft exotic Middle Eastern aromas. Americans, who once found Europe a cheap summer playground, slink around with their shrunken dollars, checking the sales and groaning at hotel and meal prices. One American tourist who checked out a London sweater store was told: "A great time to buy. We're having a half-price sale." But even at 50 percent off, the brand name that was priced was more expensive in London than at home in San Diego. Europe is anxious, despite the outward prosperity of the cities, the immaculate look of the countryside, and a marked reduction in international tension which should betoken a lasting peace. In Britain the long reign of Margaret Thatcher, that Iron Lady of politics, is over. That occasions both relief and uncertainty; relief because the country is free of Mrs. Thatcher's increasing bossiness, but uncertainty because it is left without her authority and direction. John Major, the new prime minister, is well-liked. In the Gulf war he exhibited the kind of genteel gutsiness that the British prize. He has weathered Irish Republican Army terrorist attacks with unruffled demeanor. The British like his sense of humor and his lack of self-importance. He is their kind of chap. But he does not, of course, have the charisma of Thatcher. Says one Western diplomat: "He's doing alright, but not great." If new political leadership is causing ripples in Britain, the future direction of Germany looms as a question mark over all of Europe. German reunification is proving more difficult than had been expected, and much more of a challenge than Chancellor Helmut Kohl had suggested it would be. East Germans are not finding the jobs and instant prosperity they had hoped for. West Germans are having to pay a much higher price than they had expected for integrating the economies of the two Germanys. Beyond this, Europeans ponder whether the Germans have really decided their future direction. Will they look west, or east? Which brings us to the other great concern of Europeans: the future of the Soviet Union, and to a lesser extent, its former East European satellites. Soviet leader Mikhail Gorbachev was in London for the economic summit last month, telling his tale of intended reform, looking for capitalist partners in attempting to rebuild his Marxist-destroyed economy. The Western leaders were cordial, encouraging, but not completely convinced. "Show us," they said, "that you're serious about developing a free-market system, and then we'll talk about specific help." The world is uncertain about the Soviet Union. Is it really on the path to reform? Will Gorbachev survive politically? Will pressures from its minorities and republics cause it to break apart? Will the military reverse the apparent course toward progress? And what of Eastern Europe? In Poland, Hungary, Czechoslovakia, peoples have set their faces against the old communist regimes. But the transition to a free-market economy is difficult and far from instant. Many hopes have been dashed. Resentment and impatience flare. The story is still harsher in Romania and Bulgaria. Refugees from the former Eastern bloc are trying to cross into Germany and on to other West European destinations. Small wonder that although there has been much progress toward democracy and peace, Europe this summer looks at the future with a mixture of hope and uncertainty.