I have some reservations about Mensch being the Limey and will continue to stick with Steele as he wasn't fully outed until Jan 2017 by the WSJ. CNN had previously all but revealed who he was by releasing his nationality and this was picked up on by various papers over here - and yes the WaPo too.
If you remember he left his home (ran away) just hours before his name was published, so in effect would be "scared shitless".