The succession of Mikhail Gorbachev to power in Moscow has thrust British Prime Minister Margaret Thatcher and a handful of other political figures here into the spotlight as the only western leaders to have had substantial contact with him.

Thatcher is also the European leader closest to President Reagan. Now her five hours of private talks with Gorbachev, during his week-long visit to Britain in December, probably also has put her "in on the ground floor" with the other of the world's two most powerful figures, as a commentator here put it.

Gorbachev's visit to Britain, his first extended foray into a major western country, now looms larger than it did at the time. After her meetings with Gorbachev, Thatcher told a TV interviewer: "I like Mr. Gorbachev. We can do business together."

It was a line that caused some consternation in Washington, for fear that Britain might be too susceptible to outward appearances of the younger and more modern Soviet politician.

But Thatcher stressed then, as did her aides tonight, that both leaders firmly believe in their own systems "and we are never going to change one another," as Thatcher put it in December.

Today, Downing Street quickly announced that Thatcher would attend the funeral of Konstantin Chernenko, just as she had attended the funeral of Yuri Andropov last year. Thatcher sent a message of condolence to Moscow and of congratulation to Gorbachev.

David Owen, the leader of the opposition Social Democratic Party who also spent a few hours with Gorbachev, said:

"I think he is bright, technically able, able to absorb the new concepts of government and science. I think that's a great help to us because many of these issues are only susceptible to a pretty sharp intellect.

"And if you are going to get some shift in position, particularly in arguing with the military in the Soviet Union, who are very powerfully entrenched, then the leader has got to be pretty capable, pretty effective."

David Steel, leader of the opposition Liberal Party, said, "Judging from my own meeting with Mr. Gorbachev, I think there is a greater sense of flexibility, of openness, of a willingness to argue and counter-argue rather than simply the wooden presentation of the standard line which has been the case with the older leaders."

Neil Kinnock, leader of the main opposition Labor Party, said Gorbachev was "sharp, straight, bright, and he enjoys an argument. That certainly was my experience in my 3 1/2 hours with him. What I liked is that he could take it as well as give it, and I think that bodes well for future developments, if we've got someone whose willing to put his cards on the table."

A senior British specialist said his colleagues "squeezed the lemon pretty hard" in terms of analyzing Gorbachev's performance here.

At this early stage, the general assessment is that Gorbachev will probably seek to make the Soviet system work better, try to modernize it, but is unlikely to change it fundamentally because he is a product of it. "The big question is, 'Does he have the iron in his soul to run that system or is he too much in the modern style?' We just don't know," one said.

The visit here in December, however, was widely viewed in the British government as having gone well. It was said that while Thatcher clearly got along with Gorbachev, she was also able to convey the sense of western resolve and conviction that Gorbachev had to see firsthand.

Gorbachev, in turn, impressed those who dealt with him close up as "very much his own man, a personality, a confident operator who debated and didn't rely on the usual scripts," according to one who attended a number of the meetings.

Another top specialist said Gorbachev demonstrated "a masterful hold over quite a few subjects," although he also appeared restricted in what he could say about U.S.-Soviet relations and Soviet foreign policy.

Gorbachev is viewed as a person comfortable with modern technology and management and undoubtedly interested in western approaches to these matters. Yet one British specialist said that western technology works in the West but not so well in the centralized Soviet system and that the need is greater for new ideas in governance than for new technology.

"If they remain as hidebound under Gorbachev as they have been," he said, "the Soviet Union still will not make an impact on the world scene anywhere near the extent they should. They are basically a one-legged giant," he said, with that one leg the military.

British officials said they knew of no European attempt to urge President Reagan to go to the funeral, assuming he would not attend because he did not go to Brezhnev's or Andropov's. They said Thatcher's decision was virtually automatic, having been discussed during Chernenko's long illness, when there were no objections raised to her attendance. She said today that Chernenko's death "deprived the Soviet Union of an experienced leader."