Hi everyone.
With FreeBMD I've checked the whole UK 1862 - 1933 for the death of:
Annie Susan / Annie / Susan
Urwin / Irwin / Cullmer / Culmer / Unwin
Nothing I could find in anywhere near the right time-frame, except:
Q1 1912 Southwark, Susan Unwin, 74, 1d 125 (she would have been born ca 1831 whereas Annie Susan Cullmer was baptised at Dover in 1833). Worth a look?
I also Googled her as a missing person, but only websites for present-day hunts for missing persons came up. If she was in an asylum / prison / convent etc she would have been picked up on a census.