"the government would just keep on going, using the same budget from last year."
Not unless things have changed since the '80s. One of the principles then was that if you don't spend your budget this year you don't get it keep it till next year and when you put in next year's estimates they're likely to be chopped back because clearly you didn't spend this year's money. So as the end of the financial year approaches you spend up on stuff you'll need for next year (microscope slides and cover-slips were two of my stand-bys along with methylumbelliferyl phosphate).
One variable factor was the over-winter spending on road clearance so if there was a mild winter there was a lot left over to spend. In the Northern Ireland Office this was known as the Spring Sales.