Most of the ones whose complaints get into the media have links. E.g. members of Labour Friends of Israel like Margaret Hodge who went on a junket to Israel specifically to apologize to Isaac Herzog (he who said "race mixing between Jews and non Jews is a 'tragedy'") about the UK Labour party's "anti semitism issues".

It's frankly quite disgusting the level of Islamophobia some of these people are endorsing simply because they consider it politically expedient to throw their support behind a foreign government.

It's not that anti semitism doesn't exist at all in the Labour party. It's just vanishingly rare and literally nobody - NEVER MIND a leader - has said anything close to as disgusting as what Isaac Herzog or Boris Johnson has said.