I believe the CAERNARVON AX, a tier 8 heavy, was available as the final tank available on the advent calendar.
I have the tank, it was also a mission reward tank earlier in 2018.
You can pick it up for a whopping 10,500 gold or from $47 USD to $127 USD.
Not sure I would buy this one again. I don’t play the British heavy tank line and I didn’t really enjoy the tank myself.
Still, I think it’s a reasonably good tank and may be of interest to folks playing the Brit tech tree.