He is not rich. He is not politically connected. He is not a contributor to an American presidential election campaign. And he is a highly regarded career diplomat.

Raymond Seitz, 50, thus, would appear a most unlikely choice as U.S. ambassador to the Court of St. James, as the London post is known. It is a job that traditionally has gone to a wealthy, political contributor.In the past, the London embassy has been studded with the names of famous American families: Adams, Mellon, Lowell, Kennedy, Harriman, Whitney, Annenberg.

The past three ambassadors have been Republican Party fund-raisers: John Louis, Charles Price and Henry Catto.

But the appointment last week of Ray Seitz, a tall, outgoing, knowledgeable professional, has been hailed by people like Theodore Wilkinson, president of the American Foreign Service Association, who have long lobbied for career diplomats being appointed to major foreign posts.

Of the 138 American embassies abroad, two-thirds are filled by career diplomats. But these are confined largely to Third World countries where the living conditions are difficult. The high-profile, comfortable Western European and Scandinavian countries are usually filled with political appointments, both by Republican and Democratic presidents.

The appointments are often embarrassing: Nominees have failed to know the name of the capital to which they are being assigned; a prospective ambassador to Singapore made anti-Chinese remarks, failing to realize the country is mostly populated by overseas Chinese; a Reagan appointee to Austria got involved in a public love affair with a prominent hotel owner; a Scandinavian ambassador was picked up for public drunkenness.

One justification for sending millionaires to plum European embassies is the high cost of entertaining - which comes out of the ambassador's pocket when the budget invariably is exceeded.

But Seitz's appointment suggests that a diplomat with a comfortable salary (a reported $115,333) is nevertheless suitable for the job.

Seitz has an impressive resume: He served as deputy chief of mission in London from 1984-89 under Price, who estimates that he spent $200,000 a year from his own pocket to pay for the heavy entertainment budget. (The State Department allows the entire embassy an entertainment budget of $159,300.)

Before that, Seitz was executive assistant to Secretary of State George Shultz, and most recently was assistant secretary of state for European affairs, which included the task of overseeing German reunification. In that job, he developed close ties with Secretary of State James A. Baker III, becoming one of the few career diplomats with access to Baker's inner circle.