What the month-long campaign for Britain's national election this Thursday has lacked in length, compared to interminable American campaigns, it has more than made up in numbing density.

Each morning begins with lenghty newspaper and radio reports of the previous night's political speeches. Then comes "election call"-a kind of "sports call" for policies fans-in which listeners from around the country phone in questions to a candidate from one of the three major national parties. Moderator Robin Day, who is to British elections what Bert Parks is to the Miss America contest, can also be heard and seen on radio and television at various other times on most days questioning candidates himself or moderating debates.

Meanwhile, Prime Minister James Callaghan of the Labor Party, Conservative Party leader Margaret Thatcher and Liberal Party leader David Steel are holding their daily press conferences, which create more election news for the afternoon newspaper and evening television news broadcasts.

After the evening news come the first of several special election reports each night, featuring more candidate debates and reports from the scores of "crucial" marginal districts that seem to abound across the land.

At 9 p.m., smack in the middle of the feature movie, there is a "party political broadcast" on all channels in which the major parties present commercials in 10 minutes of free air time each night.

After a precious few minutes of nonelection entertainment come the late-night news shows, expanded during the campaign to supply still more election news. They are followed by still more in-depth election specials, including one, featuring 10-minute excerpts from speeches by leading candidates of each of the major parties, that has the potential for reducing sleeping pill sales.

It is all so terribly serious and so firmly focused on the issues that a reporter travelling Britain during the campaign heard farmers around Cambridge and white-collar workers and factory hands in the industrial Midlands conversing with candidates about Rhodesia, the Common Market and fine legal points of secondary picketing.

There is little of the mudslinging and charges about questions campaign contributions that so enliven American elections. This is due in part to everyone's knowledge and acceptance of the fact that the Conservatives get most of their money in donations from businessmen and the Labor Party from unions. In fact, many Labor candidates are fully "sponsored" by individual unions.

ONE OF THE MOST telling signs of the deadly seriousness of this campaign was the announcement by the tabloid Daily Mirror last week that it was removing nude pinups from its third page until May 4 to "use this page to bring you vital facts about the election." It's chief competitioner, Rupert Murdoch's Sun, did its civic duty its own way, by putting a nude on the front page, with a powdered wig on her head, to draw attention to its election coverage.

In a country where political partisanship in the news columns is an honored tradition, another morning tabloid, the Daily Mail, has won the competition for the most outrageously biased coverage hands down with a remarkable week of pro-Conservative progaganda.

On Tuesday, there was a featured interview with Margaret Thatcher's daughter telling her mother is witty, warm and wonderful with children.

On Thursday, the entire Daily Mail front page was devoted to "Labor's Dirty Dozen" of "12 big lies they hope will save them"-complete with Tory refutations-all of it taken directly from a Conservative Party handout.

And on Friday, there was a frontpage blowup of the offhand remark that former Labor prime minister Harold Wilson made to a Mail reporter that even his wife might consider voting for Thatcher because she is a woman.

ONE OF THE BIGGEST problems that faced Fleet Street this week was what to do about the formal election address to journalist Auberon Waugh, who is running as the self-proclaimed Dog Lovers' candidate for Parliament against former Liberal Party leader Jeremy Thorpe in North Devon.

The problem is that Waugh makes requent and caustic reference in his address, which candidates here routinely pass out in election literature to voters, to the fact that Thorpe is to stand trail shortly after the election on a charge of conspiring to murder a man who said he once was Thorpe's lover.

Waugh also refers to Thorpe as "a man about whose attitude to dogs-not to mention his fellow human beings-little can be said with any certainty at the present time."

British law severly restricts what can be published about a pending criminal case beyond what is testified to in court. An appellate court ruled that Waugh coul not publish his address because it contained "serious innuednoes imputing guilt to Mr. Thorpe." The decision was made, however, after part of the address appeared in The Guardian, one of Britain's serious newspapers, and all of it in Waugh's regular column in the political magazine Spectator. CAPTION:

Picture, JEREMY THORPE, Thorpe's opponent refers to him as "a man about whose attitude to dogs-not to mention his fellow human beings - little can be said with any certainty at the present time."