Book #16 of the Papa Pontivy series.
The term Travelling Executioners was used for the muscle men of Stalin's O.S.2 department, tasked with tracking down and eliminating any "enemy of the regime". When Khrushchev disbanded the organization, one of its leaders transferred himself to London to take over spying there. When he goes after a young diplomat with a honey trap, Papa Pontivy is asked to step in to help bring the spy cell down.
Since Soviet spies are in London, the involvement of Marshall of the Special Branch is easy to understand.