(People told me to bring this question here from the tank subreddit so I thought I might as well ask any British military history experts around here)
From their original designs during world war 1 British tank designers had a huge variety of designs, with a wide variety of names, as interesting and original as "medium tank Mark III" or "Light Tank Mark II" (they also had designs with actual names such as valentine or Matilda) however, sometime during the World War 2 period, their naming convention shifted to one very strict seeming rule, any tank that made main production had a name starting with a "C". Wartime designs such as the Churchill or the Cromwell, late war/post war designs such as the Comet or the Centurion, and cold war to modern designs such as the Chieftain or the Challenger.
All of these designs are starting with a C, but I can't find anything that explains why they shifted to such a singular naming convention, especially one that doesn't make as much sense as the Germans naming tanks after big hunting cats, or Americans naming tanks after significant military figures.
The British ran into a problem: everything had a very similar name. You had the Bomb Projector Mk.III and a 6-pounder Mk.III installed in Infantry Tank Mk.III. And then they started getting American tanks, so you had the Medium Tank Mk.3, which was distinct from the Infantry Tank Mk.III and the Cruiser Tank Mk.III... except the M3 was used in a cruiser role, so you do sometimes see it called a Cruiser Tank M.3. Also, there was the Canadian M.3 (also called M4C, Medium Tank M.3 Canadian, Anglo-American Cruiser, etc), which was not a Medium Tank M3 at all. Naturally, this caused a great deal of confusion.
There was also a situation of subtypes of tanks. For instance, you had the Infantry Tank Mk.III, but there were lots of different kinds. So they were called Infantry Tank Mk.III*, Infantry Tank Mk.III**... as you can see, this would get quite bothersome to write after a while. In the end, there were 11 different marks of Infantry Tank Mk.III gun tanks alone, and counting those stars would have been a challenge.
In the fall of 1941, the British decided that they had enough. Tanks that were still in production were given more easily palatable names. American tanks were named after American generals (the Americans themselves reluctantly agreed to use the name Sherman in unofficial capacity, but continued to refer to their tanks by codes), Canadian tanks were named after animals, self propelled artillery was named after clergy.
Some artillery was given A-names, for example the Alecto (95 howitzer on the Light Tank Mk.VIII/Harry Hopkins platform), Achilles (American M10), Archer (17-pounder on the Valentine chassis), and Avenger (17-pounder on the Challenger chassis).
AA guns were called after reptiles: Skink (AA tank on Canadian Grizzly chassis), Copperhead (M19 twin 40 mm), etc.
There was another convention, for armoured cars. The M8 became the Greyhound, the M38 became the Wolfhound, etc.
Sources:
Roger V. Lucy Canada's Pride: The Ram Tank and Its Variants
National Defense RG 24 C-2 vol 12181 1/AFV GEN/1
National Defense RG 24 C-2 vol 12239 1/3 M 10/1
Canadian Military Headquarters, London (CMHQ), Files Block No. 55 - 5772