I think att stations Sutton-Wimbledon-Streatham (exclusive) are managed by FCC.
To be nearer to Mitcham than to Sutton, I think it must be north of Mitcham.
Anywhere with a direct service to one of the Mitcham stations has a direct service to both of them, so it either has a direct service to Haggerston or Sutton. I believe that all stations with direct services to Haggerston are managed by Borris, with the possible but irrelevent for the clue example of West Croydon.
So, we are looking for somewhere with a direct service to Sutton. Gipsy Hill?