Parked at the Golf Clubhouse, was a 1957 Cadillac Fleetwood. The series was set in the early 1950s 1953, and Caddies were distinctively different in that era, but this writer has not seen enough episodes to be sure it reaches the late 1950s. In any case it is hardly imagined a titanic tank like that Caddy (it may have been RHD but this writer couldn't see) sharing narrow country roads with Penelope's Rootes Talbot, or Inspector Mallory's Wolsey. Or for that matter the Padre on his bike.
At 26:41-43, you can see the "corpse" of Raylan Reeve breathing.
When Father Brown was looking at the bananas, they were obviously of the Cavendish cultivar, which was not available in the UK at this time.