One purported photo of Parvez - who one British tabloid dubbed "Hungry Hamza" for his frequent posts about food - shows him bearded, armed and eating ice cream in the Iraqi city of Mosul, which was once held by Isis (Islamic State).
But he also released threatening videos urging British Muslims to abandon their lives and join the fight, in what he called the "golden era of jihad."
Parvez is just one of many former Isis fighters and supporters now in custody and claiming contrition as their fate hangs in the balance.
"Regret is not even the word. If there was a stronger word, I would use it," Parvez, appearing gaunt and clad in loosefitting prison garb, said of his decision to join Isis five years ago.
He spoke to the Washington Post at the prison in northern Syria, in the presence of armed guards.
Parvez said he lost 30kg during his five years in the caliphate.
"What happened was completely horrific," he said of the group's tactics. "I don't think we should ever allow anything or anyone to take Islam as a religion and for them to manipulate it . . . and to shape it the way that they shaped it."
But such apparent remorse is unlikely to sway Britain's Government, which says it will not repatriate British nationals who travelled to join Isis. Thousands of foreigners and their children remain detained in Al-Hol camp in northern Syria, having fled the fighting in Baghouz.
Few Western governments have said they will take back their citizens who flocked to territory once controlled by Isis. The group is accused of carrying out horrific atrocities across both countries, including mass executions, sexual slavery and widespread destruction of ancient sites and artifacts.
Last month, Britain stripped London-born teenager, Shamima Begum, of her British citizenship after she emerged from the caliphate's final scrap of territory in Baghouz. She is now languishing in a camp in northern Syria, her future uncertain.
Parvez, similarly, was from an affluent neighbourhood in West London, where he attended Holland Park School, according to British media reports. He was the first British citizen known to be fighting with Isis. He lived both in Raqqa, the group's de facto capital in Syria, and in Iraq's Mosul.
In previous posts, he published photos of a ginger kitten he adopted in the caliphate and named Anbar after Iraq's largest province. At the time, his family expressed shock that he had travelled to Syria to fight for the group.
In the interview Saturday, Parvez claimed to have joined the caliphate to help build the state as a civilian - despite his allegiance to the group's violent ideology.
He says that he worked as a caretaker of mosques under Isis control but soon became disillusioned with Isis' brutal codes.
"Coming in, you very quickly find out it's not how it's [portrayed]," he said of the utopian version Isis members promoted online.
"The things we found out, we found out the hard way," he said, and claimed that his superiors forced him and others to fight on the front lines.
The "big fish" of Isis, he said, "stole the money and made their way out a very long time ago."
He referred to the civilians living in Baghouz, an encampment of tents that made up the Islamist militants' last stand, as "the crumbs of Isis".
"There was a complete collapse of social and economic rule. People referred to it as Dar al-Kuffar," he said, an Arabic term denoting land ruled by nonbelievers.
"There was food, but it was only or the big fish of the Islamic State. I held children in my hands who were dying because there was no food" for the remaining civilians, he said.
Then, he added: "You don't have to tell me what city I'm in, but can I ask: Is there a McDonald's here?"
