British Prime Minister Margaret Thatcher's new economic adviser expects the British pound to fall somewhat against the dollar unless american inflation takes off at an "horrendous" rate.

Professor Alan Walters, a British economist who has lived in Washington for nearly five years, believes that Thatcher's strict monetarist polices are right, however, and that the present high value of the pound is a "vote of confidende in mrs. Thatcher by the very shrewed chaps in the international community."

Walters was an economics professor at the London School of Economics before coming to Washington in 1975 and has known Thatcher for "about 10 years." His appointment to a key position in Thatcher's office comes as the British government is under fire because of sharply rising unemployment, still-high inflation and an explosion in money growth this summer.

He said he will urge the prime minister to keep to her hard-line polices aimed at cutting public spending and borrowing and at holding down the growth in the money supply.

"the main thing is not to try now somehow to stimulate the economy to mop up unemployment," Walters said in an interview yesterday.

Although he has not yet signed a contract for his new job, he expects to leave Washington at the end of the year. He will resign for the World Bank, where he is now an economic adviser on transport and other matters, but will take a leave of absence from his other employer, Johns Hopkins University, where he has been a professor of economics since 1975.

The Conservative government's economic polices are seen by many outsiders as a live experiment in monetarism. Since Thatcher's government came to power in May 1979, it has attempted to apply monetarist prescriptions for Britain's economic ills of high inflation and low productivity.

It has advocated a steady slowdown in money supply growth, cuts in public spending and a year-by-year reduction in the budget deficit. Walters supports doing "exactly what the Conservative manifesto [the equivalent of a party platform] said."

"Obviously, I agree with the general policy, and I think its going to work," he stressed yesterday, adding "I would not be stupid enough to go back to join a ship that had a high probability of sinking."

That is just how many British commentators how characterize the Conservatives' monetarist policies. Increasingly, economists are critical of the Thatcher policies, which they believe have helped push British unemployment to levels last seen in the 1930s while doing little to restrain inflation. There is also growing political pressure on the government to modify its policies.

Ironically, it has been unsuccessful so far in bringing down the rate of growth of the money supply, which soared by 8 percent in July and August alone as a result of technical changes in money control. Sources reported that Thatcher was furious with the Bank of England over this episode. However, the negotiations with Walters were not triggered by this.

Walters believes that, given the problems facing Britain, it is "remarkable that the whole thing is getting along so well." He dismissed most of the economists who have been criticizing the government as "journalists" with too short-term a view. He argues that economists know very little about how the economy works overall and that talk about managing the economy to reduce unemployment is "nonsense."

He says that because, in fact, the money supply has grown rapidly over the last year, those who blame the current British recession on a money squeeze must be wrong.

But he adds that governments can cause inflation by overexpanding the money supply. It takes a long time for lower money growth to feed through into inflation, and "it is a great problem that the economic cycle is so long," Walters said. But he added that he has a "great belief that Mrs. Thatcher knows that," implying that he expects her to stick by her policies even as the next general election -- due by spring 1984 at the latest -- looms.

He will be the only economist on the prime minister's personal staff at 10 Downing Street. From there he will compete with the economists in the Treasury and the Bank of England who also give the government advice.

Walter's appointment could come as soemthing of a blow for the Treasury's chief econmic adviser, Terry Burns. He was thought to have been chosen personally by the prime minister at the beginning of this year for a post which usually has gone to much older men who are career civil servants. But Walters remarked that he and Burns have a "somewhat similar approach to each other."

Walters maintains an optimistic view of Britain's economic prospects. He said yesterday that it is "likely that there will be rises in output and employment at the same time as desinflation [a slowdown in inflation]."

He said that this had happened at other times in other countries and gave Chile as an example. He recently returned from Chile, where he was looking at credit markets and monetary policy. He was not working for the Chilean government, which has a very poor reputation on human rights.