In 1939 the standard British tank gun was the 2 pdr Ordnance Quick Firing gun, it was fitted to the majority of tanks in the infantry and cruiser classes. A small proportion of tanks were armed with a 3 inch howitzer to fire smoke shells as the 2 pounder only fired solid shot. This howitzer could not fire HE or AP rounds.
The 2 pdr was a small compact weapon, tank turrets could therefore be kept down in size and only required a turret bearing ring of limited dimensions. This fact would become a major issue when the 2 pdr became obsolete and a larger gun was required. The 2 pdr was a 40 mm calibre weapon firing a 1.2 kg shell, capable of a maximum penetration of 57 mm of RHA armour at 500 m .
The majority of tanks at this time had a maximum armour thickness of 25 to 30 mm. One notable exception was the Matilda, designed as an infantry tank it had very thick armour, up to 65 mm and was almost immune to any weapon possessed by the Italians in the early desert campaign.
It is probably worth mentioning the British tank design types that were produced before the war. There were light tanks, fast with thin armour for scouting, cruiser tanks which were fast with less armour to exploit a breakthrough of enemy lines and infantry tanks, heavily armoured, designed to support attacks on trenches or fortifications. These designs assumed another war, after the great war, would be fought in the same fashion. The german Blitzkreig tactic, fast moving armour and infantry supported by dive bombers, was a concept not considered by the army.