At the same time, Mr. Sunak declared that the government was earmarking 30 billion pounds (about $38.7 billion) to help businesses and workers menaced by the coronavirus.

Taken as a whole, the Bank of England and the Johnson administration effectively signaled that Britain would not hold back in confronting the dangers of the outbreak. That action stood in marked contrast to the United States, whose population is nearly five times the size of Britain’s, yet has so far approved only $8.3 billion in emergency spending for the coronavirus.

“What we have seen in the U.K. is a much more coordinated, much larger response than in the U.S.,” Mr. Goodwin said.

Ever since the global financial crisis of 2008, the world has relied on central banks and their ultralow interest rates to stimulate economic activity. Economists have warned that this was not sustainable. Cheap credit was nurturing a dangerous dependence on easy money that could end in disaster. Fiscal policy was necessary, the experts said.

Britain just heeded that call. The budget increases spending on public investment like infrastructure from a current annual pace of 2.4 percent of Britain’s total economy to 2.9 percent over the next four years, according to Oxford Economics. That should lift overall economic growth by about 1 percent over that time frame.