This gentleman and his lady friend may just be anonymous, off-the-shelf heads but they look like portraits to us.
Site: Unidentified heads (2 memorials)
SW1, Cockspur Street, 25 (about)
Ornamental Passions says of this building:
"originally built for the Cunard Line in the late 19th century (all the buildings in the street were booking offices for steamships)."
Directly below the gentleman's head, at street level, there is a narrow passage that gives foot access to Cockspur Court and then via a staircase up to Carlton House Terrace and through to the Mall. Impress your friends.