It's a mistake. News articles don't begin with "But". Someone evidently thought it looked better with the headline below the article. Originally it may have looked like this:
UK virus infections back to September levels
But the World Health Organization warned that Covid cases around the world are continuing to increase at a "worrying rate".
When the text is arranged like this the word **But** makes sense. There is good news BUT there is also bad news.
(By the way, "warned" should be either "warns" or "has warned.")