London — The sudden, unannounced presence of Prime Minister Margaret Thatcher among the 900 Falkland Islanders 8,000 miles from London had political as well as personal motives.
Mrs. Thatcher wanted to gather information first-hand with which to handle strong criticism about to be leveled in Parliament on how the war began and how victory was won.
A report by Lord Franks on the origins and conduct of the war will be high on the agenda when Parliament reconvenes Jan. 17. Eagerly awaited by opposition parties, the report has not yet been released, but Mrs. Thatcher spent a weekend reading it at Chequers, the prime ministerial country estate.
Opposition parties in the House of Commons will concentrate on how Britain slipped into a war no one wanted.
But some political observers say the opposition parties won't gain much political capital from their expected onslaught. The mood in the country, these observers say, is, ''We beat the Argies [Argentines]. Certainly somebody got it wrong to start with, but the islands are ours and that's that.''
''The point is that we won,'' as one commentator puts it. ''People don't much care about the details now. Unemployment and the economy matter much more.''
Nonetheless, politics are unpredictable, and Mrs. Thatcher is taking no chances. A major purpose of her Falklands trip is to find out for herself the status of the garrison, and the financial and practical problems of maintaining a British territory so far from home and so close to the still-unfriendly Argentine mainland.
Mrs. Thatcher has made no secret of her desire to thank British troops in person for last year's victory. However, her political foes, and even some friends, say she sees too much in the Falklands episode. The Labour Party, the Liberals, and the Social Democrats say the Falklands, while a welcome victory, are a peripheral strategic issue - and expensive beyond all proportion to their importance.
Estimates in Westminster say Britain is spending about (STR)1 billion ($1.6 billion) in the current fiscal year on bills from the campaign. By l985 the bill will have risen to (STR)3 billion.
Such figures as former Labour Foreign Secretary David Owen, argue that the tiny Falklands population simply cannot live indefinitely in a state of hostility to the Argentine mainland. They want negotiations with Argentina.
But Mrs. Thatcher rules out such talks, saying British lives were not lost so the Falklands could be negotiated away.