Chieftain as a tier 10 British heavy? Not really...

Well, I think this issue is clear now. Blachard (EU server military specialist) explains:

Source

Silentstalker: ....But then - around November 2012, in the developer Q&A thread they mentioned they weren't too happy with the FV215b (that was before the rebalance) and that they sent for some documents in Bovington, regarding the Chieftain and some other things (presumably, the holy grail of FV100).... (partial question redacted)

Blanchard: Oh I know very well, since I am the one who contacted the archives and procured the documents. For info, that was from the British National Archives, not Bovington.
Sadly, the documents didn't contain much documents on the tank technical details. Mostly stuff about financial aspect of the project, logistics and partnerships, so that option went out of the window.

The best Chieftain candidate to be introduced in-game would be this one:

A prototype recently saved by Bovington's Museum.

I will go to Bovington, and I will do research on this tank, just like I did for many others. It's a good tank, it correspond to the time period, so yes, it interest us. That's all there is at the moment. A hint of interest, and people gathering data on every possible tank that could get introduced in-game.

There is no official talk about about replacing FV215b, we are not even talking about implementing the tank in-game at all. Sorry.