Cold Comfort Farm is set 'in the near future', and various clues later on in the book indicate that this imaginary 'future' is at least post-1946. One of the major changes Gibbons envisages is change within the transport system, with commercial aviation widespread within Britain.
The 1920s and 1930s were decades of enormous progress in aviation, both commercial and civil, so it is not surprising that Stella Gibbons should imagine further developments in this field. The fictitious Belisha Bat is presumably named after the politician Leslie Hore-Belisha who was a Member of Parliament when Cold Comfort Farm was published. He subsequently became Minister of Transport from 1934 to 1937 when he was responsible for both the development of the driving test and the Belisha beacon.