British air intelligence wrere well aware of the range of German bombers in 1938, probably from Admiral Canaris and Colonel Oster. That information would have been given to Chamberlain. From the internet the Dornier was 800 nautical miles and the Heinkel even further:
Specifications[edit]
Performance
Maximum speed: 440 km/h (273 mph)
Range: 2,300 km (1,429 mi) with maximum fuel
Service ceiling: 6,500 m (21,330 ft)
Rate of climb: 20 minutes to 5,185 m (17,000 ft)
Wing loading: 137 kg/m² (28.1 lb/ft²)
Power/mass: .082 kW/kg (.
https://en.wikipedia.org/wiki/Dornier_Do_17