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