denounce

Nero[…]sent his Satellites or officers toward him, to denounce the decree of his death to him[…].

to denounce someone as a swindler, or as a coward

to denounce a confederate in crime

to denounce war; to denounce punishment

It would be possible to “denounce” (leave) the Convention altogether, but short of that, legislation to restore the death penalty would place the UK government in breach of its treaty obligations under the ECHR; it would breach international law.