The plumber who earns more than the professor is a worldwide phenomenon. But doctors, engineers, company executives and other skilled professionals here also are paid badly in relation to their responsibilities, and this partly explains the decline in Britain's economy.

The lack of incentive is such, in fact, that increasing numbers of managerial types are responding to the classified newspaper advertisements that offer lucrative jobs in the United States, Australia, the Middle East and Africa, and this brain drain is aggravating the economic decline by depriving the country of talent.

There is no "average" salary for the British professional class. But even top-scale wages here are only a small fraction of what similar positions would command in America, West Germany or France.

The most a financial director of a major British manufacturing corporation might might hope to earn, for instance, is the equivalent of $25,000 a year -- roughly one-third of the wage his counterpart receives in the United States or in Western Europe. The chairman of the board of a large department store with several branches abroad gets 60 per cent less than his Canadian manager.

Rank is an important factor in the salary ladder. An assistant professor may have to settle for an initial wage of $5,000 a year, or about the same as that paid an experienced secretary. A full professor in a distinguished university, on the other hand, can expect to make as much as $16,000 a year.

Sometimes, though, the climb up the wage ladder is slow and not particularly rewarding. A doctor in a government-run hospital will earn about $14,000 a year when he reaches the status of specialist or consultant, usually in his late 30s. But even though he may become prominent in later years, he may not make more than $25,000 a year, including bonuses and fees from private patients. In the United States, incidentally, some doctors spend double that amount on insurance premiums to protect themselves against medical malpractice suits.

It should be added that, in contrast to the United States, many senior positions in Britain carry prequisites such as generous pension plans and company automobiles, often with chauffeurs. The inspiration to earn higher wages, however, is dampened by two key elements.

In the first place, the tax structure is designed to hit the upper brackets hard. An executive earning $25,000 a year, for example, pays a tax of some $11,000 after deductions. Thus, in many cases, a promotion for a professional man or woman can mean a cut in real income.

Secondly, the wage restraint policy imposed by the government two years ago froze salaries above $14,000 a year while permitting raises below that level. The result has been to narrow the pay gap between blue-collar and white-collar occupations so that profesionals in general now earn only about 25 per cent more than skilled factory workers.

This situation prompts the question of whether British doctors, engineers, professors, managers and others in that category are less valued here than they are elsewhere. The answer, it seems to me, is complex.

Salaries are certainly one way of measuring the importance a nation ataches to its occupations. And in Britain not all wages are low by international standards.

Top British civil servants, for example, tend to be paid as well as high-ranking bureaucrats in wealthier countries such as West Germany. This suggests that Britain puts a premium on government service, both in terms of income and prestige. The economic decline here, therefore, can be attributed in part to the fact that able people prefer the bureaucracy to business and technological positions.

At the same time, though, professional salaries here are low because the entire wage level is low. The British professional classes, in short, are functioning in a country that has fallen behind other advanced industrial societies.

This reality grates British professionals most acutely. For Britain is no longer isolated, but has become a member of the European community, and the British engineer, salesman or airline pilot who observes the relative affluence of his German or French colleagues must necessarily feel inferior.

It is small consolation to these professionals, then, that they are still somewhat better paid than laborers.