BACK in the 1950s the long-suffering British voter was expected to endure TV and radio broadcasts by political parties that lasted as long as half an hour. Called "party-politicals" by broadcasters, they consisted largely of "talking head" candidates droning on about past achievements and future prospects.This year, as a general election looms (it must be held before mid-1992, but could come earlier), the parties have taken several pages out of a book that might have been written by Hollywood and Madison Avenue. They are offering television viewers some of the slickest party-politicals ever shown. The films are broadcast under a law requiring TV companies to give free air time to all of Britain's main parties. The ruling Conservatives and the opposition Labour party (out of office for nearly 13 years) have each enlisted leaders in the film industry to try to persuade voters - in the space of 10 minutes - that they deserve five years in charge of Britain. John Schlesinger, director of "Midnight Cowboy" and "Billy Liar," came to the Conservatives' aid by working with the advertising company Saatchi & Saatchi on a film shown nationwide at peak viewing time on Sept. 19. It relied heavily on what Conservative strategists call the "feel-good factor." In Schlesinger's hands, Britain under the Conservatives was a country where apparently it never rained, cows blithely grazed, commuters joyfully lept out of bed the moment their alarm clocks rang, and placid clerics umpired games of cricket on sun-drenched village greens. In the background the slow movement from Mozart's Piano Concerto No. 21 bolstered the mood of measureless contentment. One viewer was quoted in the Daily Telegraph newspaper: "That looks like a nice country. May one go and live in it?" Labour meanwhile was putting the finishing touches to its own party-political, with Hugh Hudson, maker of "Chariots of Fire" in the director's chair. Mr. Hudson, a committed socialist, gave his services free of charge. Where the Conservatives were intent on stressing their achievements and afforded Prime Minister John Major only brief exposure in the film, Hudson confronted what some of Labour's election strategists see as their party's main problem - its leader, Neil Kinnock. Three days before the film was shown on Sept. 25, a public opinion poll suggested that with someone else at the helm, Labour would be 10 points ahead of where it is now. Hudson set out to portray Mr. Kinnock as a genial Welshman, a family man, and a thoughtful politician ready for power. "I have always felt Welsh," Kinnock announces, as a Welsh male choir rams the point home. Under Hudson's direction, one of the Labour leader's most criticized characteristics - that he talks a lot - was briskly dealt with. His comments were boiled down to tightly edited sound bites, each lasting only a few seconds. In this fashion Kinnock took issue with Margaret Thatcher's contention that "there is no such thing as society." Accompanied by the pianist Richard Clayderman, he declared that Britain needs to "discover genius." At the end of the film, to the strains of music from Verdi's opera "Nabucco," Kinnock climbs aboard a helicopter that lifts him high above the gabled roof of the Houses of Parliament. The film got mixed reactions. An official at Labour party headquarters in London said that "where the Conservatives in the Schlesinger movie created a false mood, we dealt in facts." Brendan Bruce, until earlier this year director of communications for the Conservative party, described the Hudson party-political as "off-target.... They were right to focus on Kinnock, because he is a problem for them." "But they took the wrong angle.... People don't want to know that Kinnock is a nice chap. They have to be convinced that he and his team are capable of governing Britain, and the film did not address that matter." Mr. Bruce said that party-politicals were not attempts to sway voters from one loyalty to another. "The idea is to keep the waverers on your side."