Alan Dershowitz, one of America's most prominent Jews, is a man who breaks bread with presidents and occasionally teaches them. He is also a fierce partisan of Israel in the public arena; a man who argues vehemently on air and in print with politicians, colleagues on the Harvard faculty, and his former camp counsellor, Noam Chomsky (yes, at a Zionist summer camp, the two men who grew up to be professors at either end of Massachussetts Avenue in Cambridge - and take diametrically opposed view on Israel - knew each other).

I was invited to meet him for a mid-morning coffee at Brown's Hotel in London and expected an argument. My views on Israel are more Ha'aretz than I assumed his were. Instead, I met a man who said he, too, would be on the left if he were an Israeli citizen, "not B'Tselem" but more "Acri" - Association for Civil Rights in Israel. Really?

"For me the most important word in the Middle East is nuance. I actually want to get Harvard to change its motto from 'Veritas' to 'Nuance'. Veritas, truth, is very dangerous because people think they actually have it. The Torah says, 'Tzedek, Tzedek, tir dov' - 'Justice, justice must you run after.' You can't ever catch it - same with truth. Today on campuses people on the hard left and on the hard right think they have the truth and nuance has no place in the debate on the Middle East. I want to replace truth with nuance."

An eminently reasonable idea, expressed simply. But it is a little at odds with his public pronouncements on American policies that he thinks affect Israel's security.

Among the nicer things he said at the time of the Iran nuclear deal was: "I would not allow this president [Barack Obama] and this secretary of state [John Kerry] - both of whom I know well, I've known them for a long time - I wouldn't allow these two people to negotiate a 30-day lease for me. They've proved to be inept negotiators." There was also: "By his own standard, he [Mr Obama] is an abject failure when it comes to dealing with Iran." A year on, Mr Dershowitz says he has come to accept the deal.

Now retired from Harvard Law School after 50 years teaching there, Mr Dershowitz moved back to his native New York City from Cambridge Massachussetts - but not his native borough, Brooklyn. "I couldn't afford it," he laughs. He lives in Manhattan and continues to practise law, working on "transnational", not international, criminal cases.

The former professor was in London this week to give the keynote address at the annual UJIA dinner. The British charity funds many programmes in Israel, as well as oversees trips for young British Jews to give them their introduction to the country. I asked Mr Dershowitz if he thought today's Jewish teenagers were divorced from Israel.

"Divorce implies a marriage," he says. "I think young Jews have never been married to Israel, so there's a need to bring them together."

His approach to making this shidduch is unusual. "I don't want it to be a propaganda trip, I want it to be a reality trip," he says.

"The first stop is not the Kotel, the first stop is not the museum [Yad Vashem]. The first stop is Ramallah."

In his view, this is the best way to counter "misinformation" about the nature of the occupation and the settlements, because they see "a beautiful Palestinian city first" and that challenges what the press has told them about the occupation.

Perhaps. A 16-year old will have no memory of what it was like to look across the Judaean hills when there was no separation wall and far fewer checkpoints. "I want them to see the checkpoints and I want them to see why the checkpoints exist and how much better life would be if there were no checkpoints."

He hopes that the students who have this experience will be better able to argue back against the anti-Israel groups proliferating on campuses, where life-long attitudes to Israel are formed.

"I remember Barack Obama when he was a student," Mr Dershowitz says.

"And I remember what was going on on university campuses when he was a student and so nothing he now believes or does surprises me."

Mr Obama was a law student at Harvard in the late 1980s and early 90s, not a period known for great student activism, so it is not quite clear what he means. Mr Dershowitz says that there was plenty of Israel criticism floating around Harvard then. Actually, the real shift on American campuses came in the early 1980s at the time of the first Lebanon invasion and the massacres at Sabra and Shatila refugee camps.

I ask him to clarify. "Obama's not anti-Israel. He's a guy whose radicalism in the 90s was tempered and now has views that are very mainstream on the left: somewhat critical of Israel; supportive of Israel and its existence and its right to defend itself but supercritical of its occupation and its settlement policies."

Sounds like a lot of non-radical, centrist Jews I know and break bread with.

Mr Dershowitz says: "I have a nuanced view. I'm critical of its settlement policies but not its occupation policies." He believes the military presence in the territories is necessary until "belligerency ends."

As an American public figure so closely identified with Israel, I ask him about Jewish identity. Diaspora Jews are hyphenates: Jewish-Americans or British-Jews. But if someone says they are Israeli there is no need to hyphenate. Israeli = Jew. How does that affect him?

"Look, I turned down Netanyahu's offer to be Israel's ambassador to the UN. I told the prime minister, I'm an American first. My loyalty and my patriotism is toward America," he adds.

"As I've said publicly, I will always be a Jew. I know that, I will never stop being a Jew. I hope I will always be an American but that's contingent. If America ever turned against the Jews I would be a Jew both because I chose that and because they chose that for me."

America turning against the Jews is not likely, Mr Dershowitz says, but in this fraught electoral season, antisemitism has returned to politics - in the UK as well as the US. Politics takes over our conversation. "I think this is one of the most important elections of my adult life. The moratorium on Jew-hating is over. Insensitivity to antis-emitism doesn't seem to be a barrier to being elected head of the Labour Party."

Donald Trump, whom he has met, is not an antisemite but is as insensitive to antisemitism as Jeremy Corbyn. "He's a politician and he's willing to accept the support of considerable numbers of antisemites." Mr Dershowitz adds: "He's a destabilising force in a world where Israel benefits from stability."

But the one thing Mr Trump is consistent about is overturning the Iran nuclear deal. Does he not think that would a good thing?

"He can't do that. It's not gonna happen. The deal involves other countries and they are not going to undo it. Look, I was against the Iran deal but I now hope it succeeds. I'm hoping I was wrong. As I told Barack Obama personally, I would love to write a book apologising for why I was wrong about the Iran deal."

And, then, coffee consumed, Alan Dershowitz went off to the House of Lords for a private meeting.