There are many facets/definitions/"flavours" of "Zionism", some of which are so inimical to Palestine and Palestinians, that that chant is understandable (note, everybody, that I said "understandable", not "OK"), particularly when you look at Israels behaviour over the last 2 years (something else understandable but not OK).
He is supporting Palestinians, and is anti-Zionism.
Zionism is a terrorist ideology. It just hasn't been proscribed as a terror ideology, like Hama has.
It's become a ludicrous situation in UK, where anyone opposing genocide by Israel is frowned upon, or accused of anti Semitism.
And anyone supporting an organisation or a person supporting the Palestinians during their oppression and occupation is arrested.
It'll be interesting to see how his arrest develops into charging with any offence.
Meanwhile Israel continues its attacks against Palestinians in Gaza.
Israel strikes southern Gaza, accusing Hamas of 'bold violation of ceasefire'