With Room to Spare
The fact that the 40 mm 2-pounder gun, adopted in 1935, wouldn't last long on the battlefield was obvious by 1938. The arsenal at Woolwich (ROF Woolwich) began developing a new gun with a caliber of 57 mm. The first barrels for the Ordnance QF 6-pounder were ready in 1940, but the development of the gun dragged on until the year later, and mass production only started in 1942. This delay (both for the tank and anti-tank versions) led to these guns only being used in the battle for El-Alamein that same year.
Meanwhile, in 1941, before the trials of the Ordnance QF 6-pounder, an idea was raised that the new gun won't last long on the battlefield either. To be fair, the 6-pounder turned out to be a great gun, with the 50 caliber version having similar penetration characteristics to the Soviet ZiS-2. Nevertheless, the idea of beginning work on a replacement for the 6-pounder was approved. The Ordnance Committee also learned that the Germans were working on a 75 mm gun capable of penetrating 80 mm of armour at 1800 meters. This was the Pak 40, which appeared on the front line in the spring of 1942 and compensated for the thicker armour of Soviet tanks.