I am basing this mostly on their news webpage. Over the last two weeks Corbyn has been smiling, looking benevolent, or standing at a podium on the BBC news home page nearly every day. Each time the headline has been pretty positive 'labour announce that' type thing. In contrast, May has been almost completely absent and Tory headlines have been negative 'Tories shown to be wrong'. I noticed this a few weeks ago so have been screen shot-ting a couple of times a day for the last two weeks (yes a bit mad I know). I have noticed a similar trend with the Radio 4 headlines though haven't been able to check that one. The question time audience last week was unquestionably partisan.
I did wonder if the Tories had hidden May or just decided not to bother campaigning but even then I thought the BBC were meant to be impartial?