The transcript of Boris Johnson's speech available on the UK Government website confirms that he had tested positive for COVID-19.
In a video message on 27 March 2020, United Kingdom Prime Minister Boris Johnson said that he had developed mild symptoms of the coronavirus like temperature and persistent cough. He also added that on the advice of the Chief Medical Officer he got tested for COVID-19 and the results were positive.
The COVID-19 pandemic has given rise to a lot of potentially dangerous misinformation. For reliable advice on COVID-19 including symptoms, prevention and available treatment, please refer to the World Health Organisation or your national healthcare authority.