London — Here in Britain, one of the strongest allies of the United States, the US and NATO nuclear shield against the Soviet Union is being questioned as rarely before.
Repeatedly, protests condemn the morality of the nuclear deterrent. Protestors say that possession of nuclear weapons indicates a readiness to use them. And since such weapons kill indiscriminately, protestors insist they are inherently immoral.
The government of Margaret Thatcher and other nuclear advocates reply that the deterrent is actually aimed at keeping the peace and forestalling Soviet aggression - both highly moral aims.
Into the protest atmosphere comes the tacit American rejection of a Soviet offer to reduce its intermediate range missiles by half if the US abandons the cruise and Pershing II missile deployment. Shadow foreign secretary Denis Healey said in London Dec. 13 that the European peace movement would see this as a US rejection of talks promised on the issue when the cruise-Pershing deployment was announced in 1979.
The US stands by its own ''zero-zero'' proposal of November 1981 whereby the Soviets must dismantle all their SS-20, SS-4, and SS-5 missiles before cruise and Pershing II missiles are canceled. The peace movement here sees that as a disproportionately heavy burden on the Soviet side.
For the moment, at least, the shield is safe in the hands of the Conservative Thatcher government. The prime minister is committed to the US alliance and an independent nuclear deterrent.
Mrs. Thatcher retains a large majority in the House of Commons. Polls make her a firm favorite for re-election despite an ailing economy and 3 million people out of work.
But as 1983 approaches, so does the deployment of US cruise missiles for NATO north of London, part of the planned 572 NATO cruise and Pershing II missiles to counter more than 600 intermediate-range missiles in the western USSR already aimed at European targets.
As in West Germany (where 40 military bases saw peace demonstrators Dec. 12) Britain is facing protest as the Labour Party and political left, many young people, sections of the Church of England and the Roman Catholic Church, and disarmers of other kinds focus on the missiles to express concerns about nuclear weapons themselves.
Many of Europe's 11 million unemployed join protest crowds in Belgium and Italy as well. Protests are planned to escalate next year.
The Tory government here insists that the peace movement is playing into the hands of the Soviet Union. It tempts Moscow to stall negotiations aimed at removing both NATO and Soviet missiles, in the belief that protestors will delay (or prevent) NATO missiles without the need for any Soviet concessions.
In recent days here:
* Seven hundred women Dec. 13 tried to stop US buses, trucks, and equipment entering a US base at Greenham Common, Newbury, north of London to prepare for 96 cruise missiles soon to be stationed there. Police had to clear a path for the US vehicles.
The previous day, about 30,000 women with babies in strollers linked hands around the nine-mile perimeter of the base.
''For centuries women have watched men go off to war,'' ran the theme of the protest. ''Now women have left home for peace.'' Two hundred women began camping outside Gate 8 of the base 15 months ago. More than 1,000 are there now.
* Antinuclear campaigners led by the Campaign for Nuclear Disarmament (CND) are campaigning against Ministry of Defense plans to build a (STR)3 billion ($4. 8 billion) bunker on National Trust land adjoining the Royal Air Force Strike Command Headquarters near Daws Hill at High Wycombe northwest of London.
CND has also seized on US plans to set up an alternative command headquarters in Britain in time of war. Current headquarters are in Stuttgart, West Germany. Armed Forces Minister of State Peter Blaker confirmed Dec. 12 that an alternative HQ was being considered, for ''administration and logistics and movement of personnel.''
Contrary to earlier reports, Mr. Blaker said that a war would not be conducted from Britain but from a center that would move from Stuttgart to NATO HQ near Mons in Belgium.
Persistent, but so far unconfirmed reports, say the alternative HQ will be established near High Wycombe. Local protestors plan a torchlight march and Christmas vigil outside the base Dec. 17, saying that their area will become a target for Soviet missiles.
* Protestors have camped outside a Polaris submarine base at Faslane on the Clyde in Scotland for several months. Fourteen arrests were made Dec. 12 when 150 people demonstrated.
* The government hits back at the peace movement by hinting that the Soviet Union is financing the movement and that communists help to control it.
* The leadership of the Roman Catholic Church in Britain has joined US Catholic bishops in expressing anxiety about ''the questionable morality of nuclear deterrents.''
The church leaders said there was an absolute moral condemnation on any act of war directed at civilian as well as military targets. The overt threat to commit an immoral act, they said, was in itself immoral. The leaders agreed that many Catholics believed nuclear weapons were preserving ''the civil freedom in which they can profess their faith.'' But they asked for regular meetings with Mr. Pym, and the foreign secretary agreed.
* The Church of England is deep in debate over a working paper called ''The Church and the Bomb - Nuclear Weapons and Christian Conscience.''
The group, headed by the Bishop of Salisbury, John Baker, gained much publicity for the peace movement by arguing that ''the evils caused by this method of making war are greater than any conceivable evil which the war is intended to prevent . . . the cause of right cannot be upheld by fighting a nuclear war.''
The group went on to urge that Britain renounce its independent nuclear deterrent.
According to reports here, NATO foreign ministers agreed privately at their recent meeting in Brussels that opposition to cruise and Pershing II missiles will be the dominant political issue facing the alliance in 1983.
''Most of the people who support CND are absolutely genuine in their concern, '' Peter Blaker said Dec. 12.
''But . . . the Soviet Union is delighted . . . at the activities of the so-called peace movements of Western Europe, which, it feels, are going to serve its purpose.''