The British had two classes of tanks during WWII, the infantry tank and the cruiser tank.

 The British had two classes of tanks during WWII, the infantry tank and the cruiser tank. 

The infantry tank was designed to escort infantry in battle and give them direct fire support. 

Its armor was supposed to be thick and speed didn't matter as long as it was as fast as a jogging human. 

The Churchill tank was one of those such vehicles. From the start the Churchill had very thick armor, just as much as the Tiger would end up having later in the war.

 However, the armor was flat faced and therefore the "effective" armor was less than the Tiger and Panther. 

The Churchill saw improvements throughout the war however it never saw a significant upgrade to the engine. 

So as the armor increased and the cannon increased it got slower and slower till the end where its top speed was about 20km per hour. It's armor though was one of its strengths and it was known to be able to take a hit. 

During the 2nd Battle of El Alamein it was reported that a Churchill tank was hit over 80 times by Italian and German anti-tank fire and was still in the fight!