LONDON—British public borrowing fell by almost half in the first six months of the current financial year from a post-World War Two high a year earlier, when the economy faced the full force of the COVID-19 pandemic. Thursday’s official data will be welcome news for finance minister Rishi Sunak, who is due to present fresh budget forecasts on Oct. 27. These are expected to show borrowing in the 2021/22 financial year is on track to come in around £40 billion ($55 billion) below the last forecasts made in March, thanks to faster economic growth. But Sunak is expected to squeeze spending for many government departments after last year’s historic pandemic spending surge. “At the budget and spending review next week I will set out how we will continue to support public services, businesses, and jobs while keeping our public finances fit for the future,” Sunak said after Thursday’s data. Britain’s …