Well, yes, Guido Fawkes, Jeremy Corbyn could have gone to the Board of Deputies, Hertfordshire's fifth or sixth most prestigious Tory club. But it is not difficult to see why the Jewdas Passover seder, a left-wing event ("Far Left" to Guido, but then what isn't?), was more his scene. Especially since it was in his constituency. And the organisers had, you know, invited him.
In any case, what is "mainstream" about the organisations that Guido prefers? Does that just mean "Tory"? They got about 300 people to their saturation-covered Conservative Party local election rally, largely disgruntled Labour MPs and the paid staff of a foreign embassy.
They should all know better than to line up with people who, since they are also venting their spleen against Winnie Mandela, would only have black people in the house to clean it, but would never have Jews in the house at all. How people would react if they saw a 14-year-old who looked like Stompie Moeketsi walking down their street is also an infallible guide to what they really think of Jews.