No one, as far as I know, has ever denied that there is antisemitism in the Labour party. There is antisemitism, racism, misogyny and all manner of other issues in all walks of life. The point that it is the same for all political parties and worse on the right wing (See antisemitism barometer). There is also the fact that this only came to light when Corbyn took the lead and became worse when he trashed the Tory lead in the last election. The media make their own news and fools fall for it.

BTW - Analysis of the British Election Study results – the gold standard of UK polling data – shows that 63 per cent of British Jews voted for the Conservatives in 2017, while just 26 per cent said they voted Labour. In the light of those figures it is quite surprising that only 40% said they would leave if Corbyn became PM.