LONDON, Oct 21 (Reuters) - British public borrowing totalled 21.778 billion pounds ($30.1 billion) in September, the Office for National Statistics said on Thursday, slightly below the 22.6 billion pounds forecast by economists in a Reuters poll. Public sector borrowing for the first half of the financial year fell to 108.1 billion pounds, down by 101.2 billion pounds from April-September 2020 when the public finances faced the full force of the COVID-19 pandemic. ($1 = 0.7239 pounds) (Reporting by David Milliken)