LONDON (Reuters) -Britain’s opposition Labour Party leader Keir Starmer will offer to resign if he is found by police to have broken COVID rules at a gathering last year, Sky News reported on Monday without citing sources. Starmer is due to make a statement at 1500 GMT, a Labour spokesperson said. The 59-year-old, who previously served as the country’s top prosecutor, has led calls for Prime Minister Boris Johnson and finance minister Rishi Sunak to resign after they both received fines relating to a birthday party celebration thrown for Johnson by staff in his Downing Street office in June 20…