The British infantry tanks were built to serve in an obsolete role. The infantry tank was trying to fill the that tanks were used in 1917 and 1918, in support of infantry in trench warfare.
That explains the existence of the diminutive Inf.Mk.I Matilda, armed soley with machine guns. The Inf.Mk.II Matilda was armed with a 40mm 2pdr high velocity gun, almost just because the gun existed, and it did make the tank more viable as a battle tank. The Matilda had good armour, good enough that it took an 88mm APC round to knock it out. It might also be disabled by a 105mm howitzer round.
The Inf.Mk.III Valentine was conceived of as tank that was more easily mass-produced than the Inf.Mk.II and it also proved to be capable of improvement, unlike the cast-hulled Inf.Mk.II. The Valentine was based on the proven Cru.Mk.I and Cru.Mk.II designs. The Valentine eventually received a 75mm gun, before it went out of production.
The Inf.Mk.IV Churchill was the ultimate embodiment of the infantry tank concept. It had the long tread, that allowed easier movement over shelled ground, and had good armour. The Churchill eventually received a 6pdr gun and then the British 75mm gun. By that time, the tank was obsolescent, but its good armour kept it around for longer than it might otherwise have been. The infantry tank concept was dead with the Churchill. It was replaced with main battle tanks that could fire HE or AP rounds, with a superior gun. The first such tank was the Centurion, of which prototypes arrived at the front at the end of the war.