Soviet Politburo member Mikhail Gorbachev, widely regarded as the number two man in Moscow and a probable heir to President Konstantin Chernenko, said here today that there are now opportunities "for the prevention of nuclear war" that "must be used to the full, not missed."

Gorbachev arrived for a week-long visit that officials believe will be the first real chance to get a close, personal look at the man who is viewed as most likely to succeed the 73-year-old Chernenko.

"There are no types of armaments that the U.S.S.R. would not agree to see limited and eventually banned in agreement with other states on a reciprocal basis," Gorbachev said in a statement issued to reporters at the airport.

Although Gorbachev's statement is in keeping with the more conciliatory line adopted recently by Chernenko, British officials said the comments were "certainly positive" and undoubtedly part of a determined effort by Moscow to make a good impression on European public opinion in advance of the U.S.-Soviet talks.

"I would like to assure the British public," he said through an interpreter at the airport, "we have come with good will and good intentions. It is our intention," he added, "to have a frank exchange of opinion on ways to overcome the present dangerous development of the international situation and make things in the world healthier again."

The emphasis on arms control, especially Soviet interest in an agreement that would ban new space weapons, also was signaled by the presence in the 30-member delegation of Evgeni Velikhov, a physicist believed to be Moscow's top expert on space weaponry.

Gorbachev's week-long visit here, the first by a top-ranking Soviet official to Britain since the era of East-West detente began to unravel eight years ago, has aroused considerable interest in the West.

His visit comes at a time of stepped-up diplomacy by Moscow, one year after the Soviets broke off arms negotiations with the United States and before U.S. Secretary of State George P. Shultz and Soviet Foreign Minister Andrei Gromyko meet in Geneva on Jan. 7 and 8 to discuss possible resumption of some form of nuclear arms reduction talks.

Gorbachev, 53, is the youngest member of the ruling Politburo. He is part of a new generation of Soviet leaders born long after the 1917 revolution and too young to have been politically formed by the trauma of World War II.

British experts say they are anxious to assess whether there are any prospects for real change in the attitude of the potential new Soviet leader. Although he visited Canada last year and led delegations to Italy and Portugal earlier this year, his visit to Britain is viewed as the first significant opportunity to assess Gorbachev since Chernenko came to power in February.

The timing also is of interest because Gorbachev will meet with British Prime Minister Margaret Thatcher Sunday on the eve of her departure Monday for Peking, where she will meet with Chinese leaders. Thatcher will then meet with President Reagan on Dec. 22 to relate her impressions of the Soviet official.

Although Thatcher is a strong backer of a tough line with Moscow on arms talks, she has twice this year, in public speeches, warned of the dangers of an arms race in space. The Foreign Ministry is known to be especially concerned about Reagan's plans for a so-called "Star Wars" space defense against Soviet missile attack. Some specialists here believe the Soviets may stress their concerns to Thatcher on this score in the hope that she may be an influence on Reagan.

The Soviets already have some crude antisatellite weapons, but most experts believe the United States would win any competition in space arms and that the Soviets therefore are seeking to check such a race.

Although British officials are hoping to probe Gorbachev on the upcoming Soviet arms position at Geneva, they say they are not likely to be able to tell the Russian much about the U.S. position, which officials here believe is not yet decided.

In Canada and elsewhere, Gorbachev impressed his hosts as an urbane, well-educated and self-confident official, more relaxed and willing to smile than his older colleagues in the Politburo. He appeared relaxed today during his airport appearance and visits to the shrines of his revolutionary forebears.

At the Karl Marx Memorial Library, on a secluded London square where Lenin once edited the journal Iskra, only a small crowd and a lone Ukrainian protester were on hand to see him. But outside the reading room of the British Museum, where Marx is said to have written "Das Kapital," communism's guiding document, Gorbachev was greeted by about 30 demonstrators supporting Poland's Solidarity union and freedom for Soviet Jews.

The Soviet official, wearing a blue raincoat, gray hat and pin-striped suit, looked in the direction of the demonstrators but appeared unruffled.

British specialists believe Gorbachev's visit may be useful for both Moscow and London on several levels.

For Gorbachev, who is a specialist in agriculture and domestic affairs, the trip is clearly his most important foreign venture thus far. It will give him valuable experience in the West, which virtually none of his colleagues except Gromyko have, and it also may be meant to show a younger and more modern Soviet face to the West.

For the British, the visit is a chance to gain some insight into Soviet thinking on both domestic and foreign policy, to expose Gorbachev to the workings of an open society and mixed-market economy, and perhaps to lay the groundwork for some future commercial deals.

While here, Gorbachev will do some sightseeing, meet with members of Parliament, travel to Edinburgh in Scotland and visit a number of British companies, including the Austin Rover car plant and the John Brown manufacturing company. The latter visit could cause some annoyance in Washington, because the company was involved in sales of material for the Soviet natural gas pipeline that the Reagan administration sought to ban two years ago.

Officially, Gorbachev is here at the invitation of the British section of the Inter-Parliamentary Union, rather than of the British government. For this reason, he was met at the airport by leaders of the British Parliament.

Gorbachev is also chairman of the Foreign Affairs Commission of the Supreme Soviet. The British invitation sought to take advantage of that link in a move that could set a precedent for a similar visit to the United States, sources here said.