Alexanda Kotey and El Shafee Elsheikh, who were captured by the US-backed Syrian Democratic Forces (SDF) earlier this year, issued the warning in a Sky News interview.
They said they did not die on the battlefield because “God decreed” it was not their destiny.
The men, both from west London, did not deny being members of the four man British IS cell, and seemed unrepentant.
Speaking to Sky News at a detention facility in Kurdish-controlled northern Syria, they said they would wait for a court to prove they were directly part of the group that held 20 hostages and oversaw the beheading of seven.
They did not specifically condemn terror attacks in London, Manchester, Paris and Brussels.
The pair insisted they were not responsible for the violence carried out by Islamic State, in the same way that British people are not responsible for the actions of the UK military.
Kotey described the videos issued by Islamic State of people being beheaded as “propaganda” and blamed western media.
When asked if they ever considered the beheadings to be “not what they signed up for” Kotey said: “It’s a bit like if a coalition air striker carried out one of his air strikes under collateral damage or the guise of it.”
While the pair accepted being members of Islamic State, they said they joined for religious reasons.
Elsheikh said: “We are in no way justifying anything Islamic State did. It is not an obligation of me to justify or condemn.
“You being a British citizen, you don’t turn around when Britain commits some war crime and say ‘but I didn’t sign up for this’.
“You didn’t sign up for it, you were born British. Many people who became religiously aligned to the Islamic State, it is a default, just like being born into a certain country.”
The group’s ringleader Mohammed Emwazi, who became known as ‘Jihadi John’, died after a drone strike in 2015.
Kotey, described by neighbours as a “reserved, polite boy”, is believed to have attended the Al Manaar mosque in Notting Hill with Emwazi after converting to Islam as a teenager.
He left his London life and two children behind to travel to Syria in 2009.
According to the US State Department, Kotey acted as a guard for IS and “likely engaged in the group’s executions and exceptionally cruel torture methods, including electronic shock and waterboarding”.
Elsheikh came to Britain as a child refugee from Sudan and lived in White City.
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Before his transformation into a jihadist, he worked as a mechanic and was described by his mother, Maha Elgigouli, as “very clever” and a “nice boy”.
The US State Department said he had “earned a reputation for waterboarding, mock executions, and crucifixions” while serving as a jailer for IS.