It appears that his condition has worsened since announcing his testing positive 10 days ago:

Boris Johnson has been admitted to hospital due to coronavirus after suffering 10 days of symptoms including a high fever, bringing doubts about his capability to lead the response to the pandemic despite No 10 insisting it was purely precautionary.

Johnson was taken to an unnamed London hospital on Sunday after days of persistent symptoms, during which time he has been self-isolating. Last week No 10 had denied the prime minister was more seriously ill than claimed.

A Downing Street spokesperson said: “On the advice of his doctor, the prime minister has tonight been admitted to hospital for tests. This is a precautionary step, as the prime minister continues to have persistent symptoms of coronavirus 10 days after testing positive for the virus.” The spokesperson said Johnson would stay in hospital “as long as needed”.

I do not know much about British politics, but what I DO know leads me to conclude that whoever might replace him would be even more of a sh%$ show than he is.