London — For many years the House of Lords has been the Sleepy Hollow of British politics. But Prime Minister Margaret Thatcher has learned that something nasty was lurking in the lush pastures of ''the other place.'' Last week, it rose up and bit her.
As a preliminary to abolishing the Greater London Council in 1986, Mrs. Thatcher asked the peers to support a plan to cancel elections for the GLC due to be held next year.
Instead, by a majority of 48 votes, the Lords threw out the measure June 28 and hoisted a black cloud over the political future of Environment Secretary Patrick Jenkin, who had been piloting the legislation through Parliament.
The incident is rated as one of Mrs. Thatcher's worst setbacks at the Palace of Westminster in five years in office.
Ironically the Lords, probably the least-democratic upper chamber in the world, took a stand on the need to preserve democracy. The argument that prevailed was that a government could not cancel elections without damaging the constitutional rights of citizens.
The men and women who administered the rebuke to Mrs. Thatcher and Mr. Jenkin almost always debate issues with high solemnity and usually follow the Thatcher line. There are some 400 peers on the Conservative benches compared with 130 for the Labour Party and 80 for the Liberal-Social Demoratic alliance.
But Mrs. Thatcher's built-in majority deserted her in spectacular fashion, leaving Lord ''Bertie'' Denham, her manager in the Lords, angry and disconcerted.
The government decided last year to abolish the GLC and other leading metropolitan councils. Mrs. Thatcher and Mr. Jenkin feared that the 1985 local elections would give public opinion a chance to oppose the plan.
So the Cabinet agreed that the elections should not be held, and that for the last few months GLC councilors would hold their jobs by nomination. A bill to achieve this was introduced.
But when it reached the peers, one after another they expressed their misgivings. The government, however, still expected a favorable vote in the upper house.
What Mrs. Thatcher and Lord Denham had not reckoned with was a decision by many Tory peers simply to stay away from the crucial vote. At about 4:30 p.m. they retreated to their clubs and London flats to take tea.
Mrs. Thatcher's first reaction was to advise her spokesmen to berate the Lords and even to suggest the House of Commons would move into open confrontation with them.
A day later the dangers of this course of action were better appreciated when some of Mrs. Thatcher's supporters in the Commons expressed their opposition to the way the government had been trying to find a way around democracy.
There was also a threat that the Lords might bite the prime minister a second time unless she modified the government's local election plans.
Meanwhile, Mr. Jenkin's chances of surviving an expected Cabinet shuffle in the autumn are not rated high. It was on his advice that Mrs. Thatcher originally supported the plan to cancel elections.