Treason has been pretty much abolished and the laws that remain are pretty munch unenforcible.
According to the law in force, it is treason felony to "compass, imagine, invent, devise, or intend" to deprive the sovereign of the Crown
This makes pretty much any Republican guilty of treason
Under these circumstances getting a conviction for treason would be pretty difficult and rather pointless.
In case you had it in mind the death penalty for treason is not still in force.
Although even if it was - the death penalty for suicide bombers is a pretty comic idea!