Prime Minister Margaret Thatcher, according to a close aide, prides herself on realism in her decision-making.

"'Recognizing reality' and 'being realistic,'" he observed, "turn up in almost everything she says."

This past week, as Thatcher and her Conservative government continued laying a foundation for the new Britain they wish to build, reality was closing in menacingly all around them.

Rapidly rising oil and fuel prices threatened to make a mockery of their vow to reduce inflation. Large wage increases for public employees made it difficult to find ways to cut income taxes without badly unbalancing the budget. Labor unions, began to rebel against Thatcher's plans to trim the government payroll. And the world oil shortage began to have a seriously disruptive impact on Britain despite its own North Sea oil wealth.

Despite assurances from 10 Downing Street that Thatcher remains determined to make a strong early economy, the money men in the City of London, Britain's Wall Street, are uneasy.

On May 4, the day after the Conservatives, won the election, brokers celebrated boisterously on the stock-market floor and the Financial Times stock index hit a record high of 558.6, prompting enthusiastic speculation that it might soon break 600. Instead, it has steadily fallen more than 50 points to just 506.4 at the close of trading Friday.

Yesterday the major oil companies increased gasoline prices by more than 10 cents a gallon, pushing the price of a gallon to just under $2. For gasoline prices to go higher, as the oil companies warn they will soon, service station pumps will have to be altered.

In addition Mobil announced it will become the first oil company to reduce supplies of gasoline, diesel fuel, jet fuel and heating oil here below last year's levels. As the first demands for rationing were heard, Thatcher's energy secretary, David Howell, warned that considerable voluntary conservation will be necessary for oil stocks to be sufficiently rebuilt for next winter's indoor heating.

All this is happening despite the fact that Britain pumps enough oil from the North Sea to supply almost all its own needs. But nearly half is sold to other countries because Britain's refineries cannot operate on the relatively light North Sea crude alone and because it needs the foreign income for its balance of payments.

The bright side of the picture is that as North Sea oil prices rise, Britain's foreign exchange income and the government's oil tax revenues also increase.

Food prices - led by milk, bread, meat and some vegetables - also rose sharply this week, embarrassing Thatcher's government, which is dismantling the old Labor goverment's Price Commission that was supposed to investigate and, when necessary, stop such increases.

Most price increases, some of which were delayed by the Old Labor government during the election campaign, would probably have been approved by the Price Commission anyway. But the conservatives feel they bungled the timing of the increases and failed to explain everything properly to an anxious public.

Because higher government salaries, another legacy of the outgoing Labor government, threatened Thatcher's plans to include sizable income tax cuts in the new budget next month, she ordered her chancellor of the exchequer, Geoffrey Howe, to work harder to cut spending. One of his first moves was to freeze government hiring for three months. Several government employees' unions responded by deciding to stop working overtime to slow down the paperwork on all the changes Thatcher wants to make.

The most militant government employe union also has decided to sabotage Thatcher's plan to encourage private health care by ordering member nurses and manual workers to refuse to care for patients in private beds in Britain's public hospitals.

Even the business community, the Conservatives' bulwark, is sending emissaries to Thatcher's industry secretary, Keith Joseph, to complain about plans to cut government spending for business subsidies and regional development grants.

Nevertheless, Thatcher's team labors on confidently. Joseph, its right-wing economic guru, has given his senior officials a reading list to help orient them to the task of turning Britain away from socialism. It includes Adam Smith's "Wealth of Nations," Jane Jacob's "Economy of Cities," Alexis de toqueville's Democracy in America" and a number of books by British economic writers including Joseph himself and even Peter Jay, son-in-law of former Labor prime minister James Callaghan.

Thatcher, who says she needs only five hours sleep a night, has set a swift pace. Having not yet had time to move to 10 Downing Street, she begins her day at her Chelsea home listening to the 7 a.m. BBC news, visits with her family at breakfast, arrives at her office by 9.30 and works there and at the House of Commons well into the night.

Although Thatcher has said she does not read the newspapers here too closely and tries not to overreact to anything she sees in them, she did notice deep drama in one story last week that a reporter had watched her doodling - writing her initials over and over - in Commons while one of her Cabinet members was speaking.

"I'll have to stop doing that," she said later. CAPTION: Picture, MARGARET THATCHER