As I understand it, he was making a pro-Hamas speech (not pro-Palestine). Hamas is a proscribed terrorist organisation in the UK.
The UK had a particular problem with preacher Anjem Choudary [1] indoctrinating followers to be militant islamists. He is reckoned to the be linked to up to 40% of terrorist incidents in the UK (including the 7/7 attacks in London).
Following this period the anti-terror laws were made stronger and "inviting support for a proscribed organisation" became illegal. Presumably that means recruiting either directly or indirectly through speeches.
So, I can understand how this happened. But your summary really only belies your bias I'm afraid, this: "arrested a jewish son of holocaust survivors for hate speech after he denounced Israel as behaving in a genocidal manner" isn't true. Even the article you link states that he was arrested for inviting support for a proscribed organisation. He wasn't arrested for denouncing Israel as behaving in a genocidal manner.
The issue is very emotive and charged. I am not taking sides. But it's often the case that whenever anyone falls foul of laws, that have been designed to protect the people of the UK, either directly from abuse because of who they are, or indirectly via terror, then those affected complain about a lack of freedom of expression.
It seems they didn't charge him, so maybe the police overreached in this instance. I haven't seen the speech, so I can't comment, and certainly I wouldn't defend the UK police as they could certainly do with some reform. But, it's also possible that this was quite a difficult tightrope for the police to walk in terms of the law.
Some of these laws may seem draconian, but the people of this country who are not racially abusing people, or targeting people because of their sexual orientation, or gender, or inviting support for a proscribed terrorist organisation (which is 99.999% of us), will never fall foul of this legislation.
