Prime Minister Margaret Thatcher formally declared to Chinese leader Hua Guofeng Britain's willingness to sell China it Harrier jump-jet fighter plane.

Her remarks at the final session of formal talks between the two leaders during Hua's week-long visit to Britain were intended to encourage the Chinese to help speed up long-running negotiations with British Aerospace on a sales contract, according to British officials.

The Chinese are interested in buying about 70 of the fighters, but have been haggling over price, credit terms and the possiblity of bartering for the jets with Chinese exports

Thatcher also asked Hua today to help stop the massive flow of illegal immigrants from China into the British crown colony of Hong Kong. Thousands of Chinese stream into Hong Kong every week despite efforts by a beefed-up British security force to stop them on land and in the water and deport those who make it across the border.

Hua promised to see what he could do about the problem, but he did not indicate what action he might take, according to British officials.

Hua and Thatcher also approved agreements today to expand cultural exchanges between Britian and China and to inaugurate regularly scheduled airline service between the two nations.

The two leaders found much to agree on during Hua's visit here this week, particularly the threat they see the Soviet Union posing to the rest of the world. Hua, who already has visited France and West Germany on his European tour, the first by a Chinese Communist Party chairman, found Thatcher most receptive to his repeated warnings about spreading Soviet influence.

Again tonight, Hua warned publicly of this treat, which he refers to as "hegemonist expansion and aggression." He said it "must be checked if world peace and stability are to be preserved."

Speaking at another of the lavish banquets the British have held for him, this one hosted by the lord mayor of London at the gold-encrusted Guildhall in the financial center of the City of London, Hua warned that "in recent years, hegemonists [the Soviets] have shown a greater degree of aggressiveness and willingness to take risks."

He indirectly accused the Soviet Union of infringing on the independence of Third World countries in Africa and Asia, "and particularly in the Middle-East," and threatening the security of Europe.

For the second time in a major speech here this week, he said "this disturbing world situation" reminded him of Winston Churchill's 1936 warning that "we must oppose the would-be dominator at potential aggressor."

"Regrettably," Hua said, Churchill's "advice was not given proper attention and a policy of appeasement eventually plunged the world into holocaust."

Hua also has endorsed Thatcher's support of NATO's proposed modernization of its theater nuclear weapons in Europe by deploying new medium-range Pershing and Tomahawk cruise missiles.

"There is an increasing awareness among West European countries that strengthening their defenses and bonds of unity is a matter of urgent importance to the cause of peace and security," Hua said Tuesday night. "We appreciate the positive efforts of the British government in this connection."

Tonight, in an apparent reference to Soviet President Leonid Brezhnev's campaign to convince NATO to postpone deployment of the medium-range missiles and enter into negotiations to reduce the level of armament in Europe, Hua warned against "fake disarmament" proposals.

Hua added that China also is "not against detente, but it must be a genuine detente and not a sham detente for camouflaging expansion and aggression.

When not conferring with Thatcher or lambasting the Soviet Union, Hua hit London's tourist sites -- from the Tower of London, where he saw the crown jewels and the royal execution block, to Buckingham Palace, where he lunched with the queen, a rare honor. Hua, whose wide grin has flashed across television screens and newspaper front pages here this week, seemed to enjoy seeing the trappings of royalty and empire.

He also demonstrated enthusiasm for British technology during tours of British Petroleum, British Rail and Rolls Royce Engine facilities.