Its not overwhelming evidence. Its a search on google? And results showing other newspaper/media in the UK saying the BBC is left biased.
On a global scale all US media is right leaning, even CNN. Which makes BBC right of center on a global scale, but still in the center on a western society based scale.
Compared with US news media, yes, BBC is left leaning, but compared with European media in general for example its right leaning. If the western scale is made up by general European media on the left side, and general US media on the right side of the line, then BBC is pretty much in the middle of all that.. If European media were US media, they would all be considered left leaning, except far right channels in Italy for example, while if US channels were European they would all be considered far right of center.