When Iris validates names against the Companies House schema it appears to use slightly more onerous validation than Companies House themselves. In my experience, when users are presented with an error message that tells them something will be rejected, they will almost always change it without testing whether the error message is actually correct.
For example, Iris will tell you that accounts will be rejected if they use "Limited" when the company was registered as "Ltd" or vice versa. But if you ignore the warning and file a set of accounts like that, Companies House accepts them because it understands that the two versions of Limited may be used interchangeably.
I have not tested this second one recently, but I believe there is a similar issue with capitalisation, where Iris would tell you to "correct" for example "IT" to "It" when Companies House would accept any version of capitalisation. It is pretty easy to change a string comparison to be case-insensitive.