LONDON, Feb 28 (Reuters) - A total of 20.09 million people in Britain have received their first dose of a COVID-19 vaccine, official data released on Sunday showed. Britain also reported a further 6,035 cases within the previous 24 hours, and 144 more deaths within 28 days of a positive test. The latest figures meant cases over the past seven days were down 21.2% compared with the previous seven-day period, and deaths were down 33.5%. (Writing by William Schomberg. Editing by Jane Merriman)