Well this is a special case which is not clearly covered in the rulebook. Can a character wheel inside of its own unit? I've scanned the rule book and the errata, and found no clear answer. The only three restrictions that apply to "can I declare a charge" is: 1) line of sight 2) forward arc 3) the charge is actually possible. I believe it is number 3 that is in dispute here
that you can not declare the charge. I have no actual quote, as mentioned, other than that I played a tournament some time ago where I wanted to charge a single cold one master out of a cold one bus. The bus was blocked such that if I had to physically leave the unit before wheeling, I could not declare the charge with that particular master that I wanted to. But if I could wheel "inside" my unit, I could. I think the general ruling, after some debate, was that I could not
charge. You have to first leave your own unit entirely, THEN wheel. And I think the cauldron is literally the only model that is big enough for this every to be a serious issue
But! Like I said, I have no actual rule quote on this. I, too, would like to have a definite answer to this question for next time I bring out the old cauldron....
But, nice trick on your part! I also used that trick against a wood elf opponent not too long ago. He blocked my horde with an eagle - far off to the side. My cauldron charged out of the unit solo, hammering into his character bunker and running it off the table