Modern automatics are almost always more efficient. The biggest is that the torque converter has a lockup clutch during crusing, and often more gears and a near perfect knowledge of when to shift gear.
Manual has lower maintenance costs regardless. A clutch plate is about $500, an automatic transmission is between $1500 and $4000. Manual doesn't require its fluid to be changed.
Automatics in UK certainly cost more, if one doesn't ride the clutch I imagine the manual will have lower maintenance costs?