- The plight of Vietnamese "boat people" rescued at sea by the British ships has created an uncomfortable political and diplomatic problem for Prime Minister Margaret Thatcher, who is determined to tighten restrictions on nonwhite immigration into Britain.

After several days' delay that dismayed some of her own Conservative supporters in Parliament and the press here, Thatcher decided last night to allow resettlement in Britain of 982 Vietnamese refugees rescued from the South China Sea by the British freighter Sibonga.

However, under pressure from right-wing Conservative who want to keep all Asian refugees out, officials emphasized that this did not commit Thatcher's government to providing resettlement here for other Vietnamese refugees rescued by British ships or for tens of thousands more boat people who have made their way in fishing vessels and freighters of other flags to the British crown colony of Hong Kong.

Previous British governments, both Conservative and Labor, had guaranteed refugee here as a last resort for boat people rescued from the sea by British ships. The outgoing Labor government also agreed to provide homes here for 1,500 other Vietnamese boat people now in Southeast Asian refugee camps - 250 each from Malaysia and Thailand and 1,000 from Hong Kong.

Thatcher's Conservative government will honor that commitment to provide homes for those 1,500 here and also will "accept as the duty of our ship captains to pick up people stranded at sea," Home Office Minister Timothy Raison said today. But he added that the next step - Britain's past promise of resettlement here for the rescued refugees, including nearly 400 more Vietnamese boat people already aboard other British ships in Southeast Asia - is now under "urgent review."

Thatcher's government also is consulting with the United States and the United Nations to find an "international solution" to the refugee problem in Hong Kong. There are now well over 30,000 Vietnamese boat people in Hong Kong, in addition to the tens of Thousands of refugees and illegal immigrants from China who enter Hong Kong each year.

The United States has taken the majority of Vietnamese refugees who have been moved out of Hong Kong camps thus far, and Hong Kong has been pressing Britain to shoulder more of the burden. The problem captured the headlines here over the weekend when Hong Kong officials refused to let the freighter Sibonga dock and unload the 982 boat people it had rescued until Thatcher agreed to resettle them in Britain.

Britain's traditional hospitality to refugees was strained in the early 1970s by ugly controversy over the admission of 28,000 Asians expelled from Uganda by Idi Amin. They symbolized for many in predominantly white Britain the rapid growth wince World War II of nonwhite immigration from the rest of the Commonwealth, particularly the West Indies, Pakistan and India.

The then Labor government considerably restricted immigration from the Commonwealth, and Thatcher campaigned in this year's national election on the repeated promise to tighten those restrictions. Opinion polls showed that Thatcher's stand was popular with white voters.

So it was not surprising when Foreign Office Minister Ian Gilmour told the House of Commons last week that the "situation has now altered" since previous British governments guaranteed resettlement here for all refugees rescued by British ships in Southeast Asia.

"The problem is very much worse," Gilmour said. "The numbers have risen considerably. I hope you would agree it would be wrong for us to enter into an open-ended commitment which might mean enormous numbers of people coming here."

Then the press here focused on the plight of those aboard theSibonga Even the staunchest pro-Conservative newspapers and some Conservative members of Parliament urged the government to act quickly to help the refugees.Last night, Thatcher acted.

Home Office Minister Raison insisted today that the "refugee problem is separate from the rest of the immigration problem" in Britain. Some Conservative politicians here who strongly support tight restrictions on immigration have favored helping the Vietnamese refugees because they see them as victims of communist oppression.

The Vietnamese refugees already settled here have been portrayed more sympatheitically by the press than the black and Asian immigrants who came before them.

But some other right-wing Conservative politicians demanded today that no more boat people be admitted.

"Britain cannot continue to be the reception area for the tropical world," said Conservative Ronald Bell. "On this basis, there is nothing to prevent people coming close to a British ship, pulling a plug and finding themselves in Britain."