"I didn't even have a toothbrush!" he said.
Demands from creditors - chiefly Glenn chasing tens of millions of pounds and New Zealand liquidators acting to recover nearly $120m for Inland Revenue - has seen Watson plead penury in several recent court proceedings and he is now openly mulling bankruptcy.
Once New Zealand's fifth-richest man, but who has sold up his local assets including the Warriors NRL team, in recent years, he said his thinking about this drastic step has changed after his stint in Pentonville. "It's a distinct possibility ... I thought that might really f*** me up, business-wise, having a bankruptcy. But now I've had prison," he said.
He said he had spent more than $20m on legal battles with Glenn in recent years - his opponent has spent $40m - and said: "In certain philanthropic ways we're both helping the starving lawyers of the UK."
A spokesman for Glenn said ongoing actions were "about justice and holding Watson responsible for his actions and, hopefully, preventing others from being deceived".
Watson claimed action taken against him by Glenn was a "vendetta," and his company Cullen's tax avoidance was due to poor advice from law firm Russell McVeagh and the judgement debt could be recovered with a lawsuit.
Liquidators acting for Cullen reviewed Watson's complaints about his legal advice and decided they were "without merit". Russell McVeagh declined to comment.
Watson also confirmed his position as a UK non-dom since 2002, a legal position popular with Russian oligarchs, which enables him to live in London without paying any income tax on income earned outside the United Kingdom.
"We do not pay tax, as many high net worth New Zealanders would know - like [the late] Doug Myers, for example," he said.
Regarding his devastating court losses, Watson claimed to be philosophical: "Litigation is, at the end of the day, it's a toss of the coin. And the mega multimillion dollar toss of the coin? We lost. And I have no qualms about that. We did lose, but you have to understand we thought we were going to win. S*** happens."
Watson said he had already written 80,000 words of a collection of anecdotes about his life and times, and true crime tales of those he had met behind bars, and hoped to soon publish as a book and sell the film rights to streaming giants.
He said he had interviewed murderers and armed robbers at Pentonville, and also intended to detail for the first time his 2002 fight with Russell Crowe in the bathroom of a Japanese restaurant in London.
In his interview with the Weekend Herald Watson also addressed Hanover Finance, a finance company he co-owned that collapsed at the start of the global financial crisis causes thousands of investors to lose hundreds of millions of dollars.
"I really do feel bad for people that have lost money. It was just, it was just completely avoidable," he said.
Watson blamed other parties, and general economic conditions, for Hanover's failure.
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• Penitentiary, my dear Watson