I'm surprised the DfT allows Chiltern to get away with such a
negligent attitude. If they agree with and condone Chiltern's
policy, why mandate a suburban service at all?

The irony is that when the line was constructed, passing loops
were created at Wembley Hill (now Wembley Stadium),
Beaconsfield and High Wycombe, but these have all now been
taken out. West Ruislip still provides a passing loop on the Up line
and the South Ruislip arrangement provides a substantial one on
the Down line.