Britain expelled six more alleged Soviet spies today in retaliation for the "totally unjustified" Soviet expulsion of 25 Britons from Moscow last weekend.

Official sources here said that the number chosen for today's expulsion list was carefully calculated to express Britain's anger at the Soviet action but that London hoped to avoid further Soviet countermeasures. Those expelled today were a diplomat, a military attache, two embassy clerks, a journalist and a member of the Soviet trade delegation.

"We want to see this business brought to an end, . . . so that this government can get back to the business of restoring decent relations between the Soviet Union and Britain," said Foreign Secretary Geoffrey Howe of the escalating row between the two countries.

But sources said that at this point they are unsure of how the Soviets will react to what has become a game of diplomatic "tit-for-tat."

It began last Thursday with the announcement here that Oleg Gordievski, a Soviet diplomat described as the KGB's chief spy in Britain, had defected. In the same announcement, the government said it was expelling 25 Soviet diplomats, trade officials and journalists whose alleged intelligence activities Gordievski had revealed.

Two days later, the Soviets responded in kind with the expulsion of 25 British diplomats, businessmen and journalists that it also said were spies. While officials here said they were expecting some sort of Soviet retaliation, they were surprised at the number and expected it to be much lower.

"They seem to be sending some kind of signal," one official said. "They are either outraged, or responding to a good deal of egg on their face. The angrier they are, the more likely they are to retaliate" against Britain's announcement of further Soviet expulsions. "I hope they have the good sense not to."

The six Soviets named today -- who the Foreign Office said also were turned in by Gordievski -- must be out of the country by Oct. 7. They are: embassy First Secretary Evgeni Safronov; Col. Viktor A. Mishin, an assistant air attache; embassy clerks Victor Daranov and Aleksandr Yerokhin; Sergei Volovets of the Novosti news agency and Ivan Vikulov, director of Anglo-Soviet Shipping.

The Foreign Office said that the overall ceiling on the number of Soviet officials allowed here would be reduced correspondingly from 211 to 205. Britain has less than half that number of embassy staff, businessmen and journalists in the Soviet Union, which places no ceiling on them. Thus, while Britain theoretically can replace those expelled from Moscow, the Soviets cannot replace their officials here.

Both Soviet Ambassador to Britain Viktor Popov and his deputy are on leave outside the country, and first secretary Lev Parshin was called to the Foreign Office this afternoon to be informed of the latest British move.

This evening, the Soviet Embassy called the new expulsions "a provocative and vindictive action of an unfriendly nature." In a statement, the embassy said, "It is hard to discern the professed desire of the British side for better relations."

In its own announcement, the Foreign Office said it viewed last weekend's Soviet action as "an unwarranted victimization of innocent people, which the British government [is] not prepared to accept." London has denied that any of the expelled Britons were spies.

The Soviets expelled from here last week, it said, "had been actively engaged in intelligence activity designed to undermine the national security of the United Kingdom."

Similarly, Gordievski had provided "incontrovertible evidence" that the additional six Soviets ordered expelled today "had been concerned in the unacceptable activities of the Soviet intelligence services in the United Kingdom."

An official here said that the first 25 Soviets "were obviously the first line of Soviet intelligence activists" turned in by Gordievski. "These six," he said, "are a further layer."

In a television interview tonight, Howe was asked why, if the latest six were genuine Soviet spies, were they not expelled along with the first group. "All these people have been identified in the information we received," Howe said.

The new expulsions, Howe added, were not the result of "pique, but a rational defense of Britain's national interests."

The current British-Soviet difficulties come at a time when Britain has been actively, and successfully, seeking increased trade with the Soviets. Although it still runs a slight trade deficit with Moscow, British exports to the Soviet Union last year reached their highest level at more than $1 billion.

Last fall, Prime Minister Margaret Thatcher described Soviet leader Mikhail Gorbachev, who was then a visiting member of the Politburo, as a man with whom one could "do business."

Despite today's new expulsions, Howe said tonight, "We want nevertheless to get back to the business to which the prime minister and I have devoted a great deal of time and attention over the last two or three years -- improving relations between East and West, between Britain and the Soviet Union."