LONDON (AP) _ The British government cut its base rate of income tax by 2 percentage points, to 27 percent, when it unveiled its budget for the coming fiscal year today.

The budget for the fiscal year beginning April 6 probably will be the last before Prime Minister Margaret Thatcher calls the next general election, which she must do before June 1988. It was seen as designed to please voters and to help win Mrs. Thatcher a third term.

In addition to the tax cut, which should spur Britain's strong consumer spending and stimulate the economy, Chancellor of the Exchequer Nigel Lawson announced a reduction in the government's public borrowing. That should push down interest rates and help spur further economic growth.

Both moves were seen as indications that Mrs. Thatcher plans to call the election early, before their possible negative side effects - higher inflation and an increase in imports - could be felt.

The cuts have been made possible by increased tax revenues, a recovery in oil prices and the government's sale of nationalized companies.

Britain, with large deposits in the North Sea, is a major exporter of oil.

''The (economic) setting for this year's budget is more favorable than it has been for many years,'' Lawson said during his presentation in Parliament.

The tax changes will ''improve still further the prospects that we have before us'' he said.

''Everyone anticipated this one would be a 'bribes' budget and indeed that is what it is,'' said Neil Kinnock, leader of the opposition Labor Party.

Last October, Lawson announced that spending for the 1987-1988 year would total $235 billion.

Lawson reiterated today that his ultimate target for the base income tax rate is 25 percent.

During his presentation, Lawson forecast economic growth of 3 percent in 1987, up almost half a percentage point from 1986.

He predicted inflation would increase to an annual rate of 4.5 percent by summer and fall back to 4 percent by the end of the year. Inflation in 1986 was 3.4 percent.

The base income tax rate of 27 percent will be paid by Britons who earn between 2,425 pounds ($3,856) and 17,900 pounds ($28,461) annually. Lawson said this year's tax changes will save 3 pounds ($4.77) a week in taxes for Britons who annually earn 10,000 pounds ($15,900), the nation's average salary.