Subscribers to Index on Censorship got a surprise this month. Instead of the usual reports of suppression of free speech by dictatorships, the entire issue of the small but respected journal was devoted to Britain.

It reflected growing alarm among editors, broadcasters and civil libertarians that Prime Minister Margaret Thatcher's government is adding censorship to Britain's already extensive official secrecy.

From repeated attempts to suppress news about Northern Ireland to a worldwide government campaign to ban the publication of an intelligence veteran's memoirs, the list of recent controversies over media freedom has grown dramatically.

"If freedom is diminished in the United Kingdom, where historically it has deep roots, it is potentially diminished everywhere," wrote Matthew Hoffman, an assistant editor of the Independent newspaper. Hoffman edited the special issue of the Index of Censorship, a monthly journal on freedom of speech and censorship.

Thatcher insists she is trying to bolster national security, deny terrorists a platform and restore discipline to a leaky civil service. But during her nine-year tenure, Britain has gained a reputation as the most secretive of the Western democracies.

"Secrecy from the top of our society to the bottom is preventing newspapers from telling the public what is really happening," the Guild of British Newspaper Editors charged in a recent report, "Officially Secret."

The problem was underscored when Thatcher ignored calls to explain a mysterious incident in which a Cuban diplomat was expelled last week after firing a gun on a London street.

Those who felt the public was entitled to know why lives were put in danger received a stock response: The government never "makes statements on matters relating to national security."

The March shooting of three Irish Republican Army members on a bombing mission in Gibraltar also stirred the secrecy debate.

When two British television companies turned up witness' accounts suggesting soldiers deliberately shot to death the three terrorists instead of trying to arrest them, the government pressured the networks to suppress the films, arguing they would prejudice the inquest.

The networks refused and the films were screened on schedule.

Other incidents have raised concern over official secrecy:

-A police raid on British Broadcasting Corp. studios to seize tapes of a documentary about a secret spy satellite.

-A legal bid this year to stop BBC radio broadcasting from interviews with current and former members of the intelligence services.

-The confiscation of unscreened TV film of street violence in Belfast; a court order delaying screening of a film about an IRA-related trial.

-The banning of a BBC film about an IRA sympathizer that provoked a one-day protest strike at the BBC.

Overshadowing all for sheer cost and persistence is the Thatcher administration's worldwide two-year court battle to suppress "Spycatcher," the memoirs of retired intelligence operative Peter Wright.

Thatcher says she is defending a principle - the life-long oath of secrecy taken by secret agents. She has lost her battle in foreign courts, but in Britain it drags on. The Guardian and Observer newspapers are still fighting in court to publish extracts of "Spycatcher."

In Bath, southern England, bookseller Peter Marsh this month received a summons for contempt of court for selling copies of "Spycatcher" that had been imported from Ireland.

Marsh works alone and said he could be bankrupted by legal costs but added in a telephone interview: "I sell books, it's my living, and I don't see why I should be deprived of that right because of some overweening authority."

Since coming to office in 1979, Thatcher's government has used the all-embracing 1911 Official Secrets Act in at least 24 prosecutions, compared with 34 the previous 68 years. The act makes it an offense for a civil servant to disclose any information, however trivial, without authorization.

The government has unveiled a proposed reform confining the act mainly to defense, intelligence and foreign relations matters, saying this would dispel any notion it was being used to protect the government from embarrassment.

But the proposals reinforce the lifetime gag on intelligence officers and shield the intelligence services from any parliamentary scrutiny.

Some newspapers have responded by printing sensitive stories under headlines pointing out that under the new law, such stories will be unpublishable.

Novelist John Mortimer wrote in Index of Censorship: "In healthier days most newspapers . . . attacked the government of the day, whichever government it might be. Now, however, too many journalists censor themselves."