A few projects related to the T95 Medium and Chieftain tanks actually existed during the Cold War, so technically the in-game T95/FV4201 Chieftain isn't entirely fictional, or at least the idea of
Supertest: FV4201 Chieftain Proto (Initial Stats) - The Armored Patrol
World of tanks(WoT) T95/FV4201 Chieftain 3 Marks of excellence MOE NA/EU/RU | eBay
T95/FV4201 Chieftain: Overview, Stats, and Tips for WoT Players