Britain maintains its options--both diplomacy and force

British strategy to regain the Falkland Islands remains diplomacy first, and force second.

London is cool to the latest Argentine proposals. But it has by no means closed the door to them.

At the same time, Prime Minister Margaret Thatcher has one eye firmly fixed on her own government backbenchers. Their support is crucial for her--and they have made it clear they will settle for nothing short of complete Argentine withdrawal, with the British flag flying over the islands, before any negotiations can begin in earnest.

Mrs. Thatcher herself emphasized April 20 that the 1,800 inhabitants of the islands themselves must be allowed to determine their own future. She sees no such provision in the proposals presented to her government late April 19 and considered in a full Cabinet meeting late April 20.

While Mrs. Thatcher herself was reported to be opposed to the new package plan when she first learned of it from United States Secretary of State Alexander M. Haig Jr., she told the House of Commons April 20 that she regarded it as a ''stage'' in the negotiating process.

By deciding to send Foreign Secretary Francis Pym to Washington April 22 to see Mr. Haig, she underlined her willingness to keep the American mediation effort alive--even as the British naval task force continued steaming closer and closer to joining the British nuclear submarines enforcing the 200-mile sea blockade, or maritime exclusion zone, around the Falkland Islands.

The government rejects, at least for now, a suggestion by the opposition Labour Party that the United Nations secretary-general should take over Mr. Haig's mediating role. The Labour Party also wants the US to side more firmly with Britain by imposing trade sanctions against Buenos Aires.

The new proposals--relayed by Mr. Haig but described here as coming from the Argentines--were clearly not enough to cause Mrs. Thatcher to halt the task force, let alone turn it around. London relies on the size, strength, and purpose of the fleet, enlarged in recent days, to force the Argentine junta to back down.

Indeed, Mrs. Thatcher believes the task force has already forced Argentina to retreat from its militant tone at the time its troops occupied the islands April 2.

Although not officially published at this writing, the new proposals were widely believed to call for a joint Anglo-Argentine administration of the islands, with a combined police force and a council half elected by the islanders and half appointed by Buenos Aires.

The question of sovereignty would be set aside for a period (as Mr. Haig told newsmen in Argentina that it should be) and negotiated at the UN. The Argentine troops would be withdrawn, the British naval task force would turn around, and the US itself would oversee the settlement process.

If accurately reported, the proposals not only fail to specify that the islanders themselves must determine their own future, but also they would mean an Argentine as well as a British flag flying over the Falklands--something Britain has said she will not accept.

Meanwhile, Britain is sending 900 more paratroopers to the South Atlantic in a requisitioned ferry. It is converting three more civilian vessels to ambulance ships. And it is reviving its squadrons of delta-winged Vulcan bombers and changing them from nuclear to conventional attack roles, apparently to persuade the junta that bombing Argentine airfields is a distinct possibility.