There is still a ghostly atmosphere of lost empire about the British Foreign Office, symbolized by the energy-saving darkness that enshrouds the grand staircase and long marble corridors, the encrusted old paintings on the walls, and the dim, ornate ceilings.

But there also are signs of a new sense of purpose since Margaret Thatcher became Britian's prime minister last May and made Lord Carrington her foreign secretary.

Beginning with the great gamble won in Rhodesia, a newly confident and daring British diplomacy is winning renewed international attention and influence, despite Britian's shrunken power abroad and continuing economic crisis at home.

For months now, it has been like old times inside the cavernous Foreign and Commonwealth Office in Whitehall here, from which the foreign policy of a major world power and sprawling empire was once directed.

"My telephone has been ringing off the hook with people asking about Prince Charles' program in Salisbury," complained a harried diplomat the other day.

Among other pressing responsibilities, he was knee-deep in preparations for the royal ceremony granting independence to the new nation of Zimbabwe next month, made possible by Britain's achievement of a peace agreement and black majority rule in its last colony, Rhodesia.

He and other diplomats also were busily involved in the British-launched European initiative to negotiate a solution to the Afghan crisis, the behind-the-scenes British leadership of the European move to recognize the right of Palestinians to a homeland, and the vigorous British efforts to enhance Britain's financial and leadership position in the Common Market. t

"The world was ready for Britain to pursue a more active role in foreign affairs than it had in recent years," explained one Foreign Office minister. "The rest of the world recognizes that Britain still has diplomatic experience and skill."

"Britain still can have a unique and creative role in Western foreign policy," agreed a senior American diplomat here, "and Lord Carrington is filling that role."

After Carrington, a 60-year-old landed aristocrat with considerable foreign and defense experience in previous Conservative governments, became foreign secretary, he told Britons he opposed "locking ourselves away, unsure of ourselves, increasingly poor, fiercely nationalistic -- but with our sovereignty locked up in a deed box."

Instead, he has created an assertive new diplomatic role for Britain. While remaining generally supportive of the United States, Britain is moving closer to Europe and becoming more active as a policy innovator in the increasing "political cooperation" among the nine Common Market countries on major international issues.

This means combining without conflict Britain's traditional "special relationship" with the United States, a larger role in Europe, and British ties with many nonaligned countries, particularly those to the old British empire and the present British Commonwealth.

Britain believes it must still stand up and be counted with the United States when necessary, as it has in putting pressure on British athletes to boycott the Moscow Olympics.

But Britain is also demonstrating that it is just a stand-in or Trojan horse for the United States, as it has in independently suggesting the Afghan neutrality initiative, moving to support Palestinian self-determination, and spearheading Common Market resistance to what Europeans believe are unfair U.S. trading practices.

Britain and the other European allies know, Carrington said recently, that "alliance with the U.S. is the bedrock of Europe's security." But he warned that "there will be times when the judgements, and even the policies of the Common Market will diverge somewhat from those of the U.S. On other occasions we will work to persuade the U.S. to modify its policies."

Carrington also has brought to British diplomacy his own distinctive, daring style. He has instilled new self-confidence in British diplomacy, especially with his successful gamble in Rhodesia.

"No one really thought when we started down the road in Rhodesia last August that it would work," admitted a well-placed British diplomat. "Our success gave us immeasurable confidence and caught the world's attention, making other things possible since then."

Since being persuaded by Carrington to follow his advice in Rhodesia, Thatcher has given him a largely free hand to devise policy with the assistance of an unusually experienced Conservative Foreign Office team headed by Ian Gilmour, the government's foreign policy spokesman in the House of Commons and an aristocrat like his old friend Carrington.

The resulting policies are strikingly less dogmatic than Thatcher's frequently strident public statement on international issues. This has enabled Carrington to appear more accommodating than Thatcher, particularly when dealing with skeptical Common Market partners like France.

French diplomats believe Carrington disapproves of how Thatcher "dramatizes things" such as her aggressive, unbending bargaining posture in the acrimonious negotiations between Britain and the rest of the Common Market over the size of its financial contribution to the community.

"She breaks the crockery," said a French official of Thatcher, but he described Carrington as a man of integrity you can talk to."

Like Henry Kissinger, Carrington uses bluff and bravura as diplomatic tools. His bold initiatives have been purposely vague, allowing plenty of leeway for negotiation. This was the case in the Rhodesian negotiations, when even British diplomats, for all their well-known attention to detail, did not know what direction the talks and later the events would take.

The British certainly were not expecting or eager to have Robert Mugabe emerge as prime minister of Zimbabwe, something Carrington could never have sold in advance to the other parties at the peace talks or many Conservatives here. But it was allowed to happen as black majority rule evolved, and the British are pleased that peaceful independence has resulted.

Similarly, one knowledgeable analyst here described the Palestinian self-determination initiative as "more than a trial balloon but less than a plan at this stage." He also considers the Afghan neutrality initiative "primarily a tactical move" by Carrington to "close the gap" between Britain and the rest at Europe on the issue and fomulate a policy "around which all of Europe can cluster."

Carrington and British diplomats now face a major test in the negotiations to reduce Britain's large net deficit in its Common Market trading.

The argument is expected to dominate the next summit in Brussels later this month, with no compromise in sight and the French darkly suggesting that Britain might be happier back outside the Common Market.

Carrington is expected to take a larger role in trying to end this impasse, which could determine his so far successful efforts to gain a larger political role in the European Community and a more influential voice in world affairs.