What's the nearest completely unstaffed (all day, every day) station to London Charing Cross (what I take as the centre of London) as the crow flies?
It appears there are 27 National Rail stations in Greater London categorised as Category F (Small Unstaffed). But I don't think this list is accurate as some stations listed as category F (such as Canonbury or Clapham High Street) are staffed full-time.
I think the closest category F station would be Queenstown Road (Battersea), which is less than 2.5 miles from Charing Cross, as the crow flies. But I believe this is a staffed station.
Other than that, it has to be
@sharpley's suggestion of Lea Bridge, at about 5.5 miles.