Williamson made the comments on Saturday in an interview with the Sun newspaper after Britain reportedly stripped El Shafee Elsheikh and Alexanda Amon Kotey of their citizenship.
"I don't think they should ever set foot in this country again," Williamson told the newspaper. "They turned their back on Britain, our values and everything we stand for — they are the worst of the worst."
The two British citizens were members of a Daesh kidnapping cell dubbed "The Beatles" which was notorious for videotaping beheadings. They were involved in the killing of detained Western hostages.
The men were captured in January in eastern Syria by the so-called Syrian Democratic Forces (SDF), which are supported by the US.
The two captured men were the last two remaining "Beatles" members who were still at large. Mohammed Emwazi, known as "Jihadi John," was killed in 2015 in a drone strike by the US-led coalition claimed to be fighting against Daesh. Another "Beatles" member, Aine Davis, is being imprisoned in Turkey.
The US State Department described Kotey, 34, and Elsheikh, 29, as executioners and jailers for Daesh.
Kotey “likely engaged in the group’s executions and exceptionally cruel torture methods, including electronic shock and waterboarding. Kotey has also acted as an ISIL [Daesh] recruiter and is responsible for recruiting several UK nationals to join the terrorist organization," the State Department said in a statement.
“Elsheikh was said to have earned a reputation for waterboarding, mock executions and crucifixions while serving as an ISIS [Daesh] jailer,” the statement continued.
According to reports, Britain is planning to send the two extremists to the notorious US military prison Guantanamo Bay or another prison outside of the United Kingdom, where it believes the rules of the Geneva Conventions do not apply.