One of the busiest spots in London today is a cramped room above a seedy cafe near Victoria Station, its walls plastered with pictures of penguins.

It is the Falkland Islands Office, and it has both a slogan ("Keep the Falkland Islands British") and a mission: For years it has been trying to defend the interests of the 1,800 inhabitants of the remote South Atlantic islands.

Although it has no official standing, the office--funded by island sheep-farming companies and British wool importers--has become a thorn in the side of the government and in particular of the Foreign Office, which is the prime target for the wrath of its director general, retired air commodore Brian Frow.

"They are beginning to realize that we have some teeth," Frow said, alluding to the British government in a recent interview. "I told them as long ago as last September that there would be a crisis if they did not act immediately. But the government insisted that everything was hunky-dory in the Falklands."

The portly, balding Frow has been in his element since his usually sleepy office was electrified by the Argentine invasion of the islands and the departure of Britain's armada. Telephones ring constantly and Frow barks out terse advice, obviously relishing the sense of crisis.

"As far as I am concerned," he said, "I reckon we are already at war."

The office was set up in 1968 by the United Kingdom Falkland Islands Committee, a 31-member group of lawyers, former members of Parliament and others, to protect the interests of the islanders.

The office also operates the Falkland Islands Research and Development Association Ltd., aimed at promoting the economic development of the islands through tourism; the export of alginate, a seaweed-based chemical used in food processing, and eventually oil exploration.

Its leverage comes through the members of Parliament, mostly Conservatives, who have been convinced that they should try to defend the interests of the remote islands.

"I hope that the problem can be solved by diplomatic means with a show of strength," Frow said. "But if force is necessary, then force must be applied ruthlessly. We should tell them to get out, or sink their ships. There should be a graduated military response. I hope it won't be necessary, but that includes the H-bomb."

Frow's stern talk is frequently interrupted by the telephone calls, answered by his assistant, Sukey Cameron, who wears a T-shirt emblazoned with the words "The Falklands are British and beautiful." Cameron also deals with a stream of visitors asking for the latest news from the beleaguered islands and extra supplies of the T-shirts.

Frow does not mince words on the subject of the Foreign Office.

"I have no objection to going on record and saying that I have a deep mistrust of the Foreign Office, and that has not changed," he said.

The Foreign Office, in turn, is not exactly enthusiastic about Frow.

"We are not prepared to comment on what Air-Commodore Frow says," a Foreign Office official said. "He has no official standing whatsoever . . . . The crisis is entirely a matter for the British government."

But Frow says he has impressive support.

"We have among our committed supporters 320 members of the House of Commons and 150 members of the House of Lords," he said. "At the moment, we also have the support of 90 percent of the British population."

Frow has been warning for some time of the coming crisis. In 1979 he complained that the runway being built at Port Stanley was too short for long-haul airliners and predicted the islands would become dependent on flights from Argentina. In September he criticized a government decision to withdraw the Antarctic patrol ship HMS Endurance from the South Atlantic, saying Argentina would interpret the move as a lack of British interest in the region.

"The Foreign Office has often regarded me as a warmonger," Frow said. "I have been accused of exaggeration. When I said that a government was liable to fall over the Falkland Islands issue, people said, 'Don't be silly. It's only 1,800 people.' I think my point is being proved."