‘Initial trials with the cannon were not encouraging, with frequent stoppages caused by the wing’s flexing while manoeuvring during an attack. An asymmetrical stoppage would result in the aircraft being slewed from its course due to the recoil of the operating cannon and thus spoiling the pilot’s aim. The first recorded operational sortie by a cannon-armed Spitfire took place on January 13th 1940 when A Heinkel 111 was attacked. Hits were observed before the cannon jammed, but they were not enough to destroy the Heinkel, which was finished off by an aircraft with standard armament.’

From Robert Humphreys’ The Supermarine Spitfire: a comprehensive guide for the modeller.

It seems that 19 Squadron exchanged its cannon armed Spits ‘during the Battle of Britain’ (doesn’t say when) at the request of the CO. Modifications to the 1b (so called since March 1940) continued and 92 Squadron were issued with a revised, cannon+machine gun armed 1b in November 1940.