I'm a little undecided about this area. I have seen a high resolution photo of this view, and those arched windows do not look like mirrors. It just looks like a plain white surface.
Also, this photo is from the Olympic. Originally on the Olympic this was a grand staircase foyer that looked out onto the promenade. It looks to me as if the paneling was reused when this area was created into two cabins, and as the paneling had windows, that was reused as well, however it doesn't look like mirrors were installed there.
Since on Titanic this area was changed to two cabins before her mainden voyage, I think the paneling may have been done without those windows. The same applies for Britannic.