Britain's foreign secretary, Lord Carrington, announced tonight that the 14-year-old order imposing British economic sanctions on breakaway Zimbabwe-Rhodesia would be allowed to expire on Nov. 15.

But British officials called it a largely "symbolic" action because the most important sanctions -- bans on British trade and financial transactions with Salisbury -- remain in force through other laws, regulations and executive orders. They also said Britain would not be abrogating United Nations' sactions against trade with Zimbaabwe-Rhodesia.

The officials also emphasized that the move did not in any way constitute British recognition of the present biracial Salisbury government of Prime Minister Abel Muzorewa. But it appeared to be a signal of reward to Muzorewa for agreeing to Britain's plan for achieving black majority rule in and legal independence for Zimbabwe-Rhodesia.

It also avoided for Prime Minister Margaret Thatcher the need for force through Parliament, over the objections of some right-wing members of her own party, renewal of the 1965 sanctions order if the Rhodesian settlement conference here had not produced a peace agreement before the sanctions order expired Nov. 15.

Finally, today's move puts added pressure on Patriotic Front guerrilla leaders Robert Mugabe and Joshua Nkomo to join Muzorewa in agreeing to the British transition plan for a cease-fire in their war and the holding of new elections.

Mugabe and Nkomo reacted angrily to the sanctions announcement and to Thatcher government efforts to rush through Parliament this week legislation to set up the British transition government in Salisbury if a peace agreement is reached.

Carrington explained in a statement today that this was being done because the conference was "moving towards a conclusion" and Britain wanted the enabling legislation in effect before a settlement here.

But Patriotic Front spokesmen accused the British of trying to railroad the guerrillas into a quick agreement despite their repeated reservations about a short election campaign period, the need for an outside force to police a cease-fire, and the presence of "undesirable elements" in the Salisbury bureaucracy, police and judiciary during the transition period and election campaign.

And despite considerable movement by both the British and the front on these and other issues last week, front spokesmen disagreed today with Carrington's assessment that "we are very close to a settlement."

Mugabe and Nkomo today promised Carrington they will reply to the British transition plan Thursday.

Both the British and the Patriotic Front welcomed the arrival here on Thursday of Zambian President Kenneth Kaunda, who is expected to try to mediate the outstanding issues in meeting with British, guerrilla and Commonwealth leaders there.

Washington Post correspondent Jay Ross reported from Lusaka :

"This is possibly the most important visit Kaunda will make to London," a Western diplomat said, referring to Zambia's increasing need for a Rhodesian settlement. Zambia, one of the five so-called front-line African states supporting the Front, has come under increasing military and economic attack from Zimbabwe-Rhodesia.