The British Labour Party was formed in 1900 before World War I, and if there has been one political movement in Britain that has stood rock solid against all manifestations of anti-Semitism in British life (perhaps more than any other group), it is the British Labour Party, and first and foremost its left wing.
This is consistent with the traditional stance of the left wing of the British Labour Party, which can be broadly defined as anti-racist, anti-fascist, and, above all, anti-imperialist. The latter has been important in a country like Britain—historically the preeminent imperialist power—where imperialism was justified in racist terms. British left-wing anti-imperialists, who are mostly drawn from the working class (a fact which gave British anti-imperialism a strong class-conflict character) strongly were predisposed to be anti-racist.
Not surprisingly, British Labour left-wing anti-racism extended to staunch opposition to anti-Semitism, which is, of course, the reason why so many British Jews were drawn to the Labour Party in the first place.
The key point to understand about Corbyn is that it is from within this left-wing, anti-imperialist Labour tradition that he comes. His parents after all first met each other in the 1930s attending a rally in support of the Spanish Republic at the time of the Spanish Civil War. He has been loyal to the traditions of the Labour Party’s anti-imperialist left ever since he began his career in politics, as his long record of opposition to all the West’s interventionist wars shows. Needless to say, that includes strong and consistent anti-racism and the opposition to anti-Semitism which goes with it. Strikingly, Corbyn’s acceptance speech following his election as Labour leader included a declaration of support for refugees.
He is pointing at the fascists and at Apartheid inside Israel, and they don’t like it, so they attack him.