Britain's month-long general election campaign ended today with Conservative Prime Minister Margaret Thatcher calling on voters to grant her a "decisive mandate" for another term.

The overwhelming likelihood is that Thatcher will see her hopes fulfilled Thursday. Last-minute opinion surveys show the Conservatives maintaining their strong lead over the opposition Labor Party and the new Liberal-Social Democrat Alliance. The main question seems to be how large Thatcher's victory margin will be.

Opposition spokesmen sought to put the best possible gloss on the chances for an upset. Labor leader Michael Foot said that the party's own national canvassing shows "that the Labor vote is not only holding up very well but is in fact increasing."

Liberal leader David Steel said the apparent boost in support for the Alliance in the campaign's later stages demonstrated that a meaningful third force in British politics had been established.

The long-term fortunes of the Alliance probably will not be decided by the outcome of the election, experts believe, because it is unlikely to get many seats in Parliament. Despite Foot's professed optimism, most Labor supporters were preparing for a substantial defeat.

One imponderable in Thursday's balloting will be the impact of the country-wide redistricting since the last election in 1979.