Hi goldcorn, if you are fairly sure your dd isn't going to go back to school soon then it seems to me that an official change of status could be very helpful, if that is what she wants right now.
I don't have experience of school refusal with my own children, but I have heard that for many young people, simply being allowed not to go to school is not sufficient to relieve their anxiety. The prospect of being made to return to school still looms, together with the feeling that the child somehow "ought" to be at school. Deregistration to be home educated can be a turning point which gives her the breathing space to relax and eventually figure out what she wants to do in the longer term.
Have you mentioned home education to your daughter? What does she think?
The education welfare officer may or may not have any knowledge of home education and it is possible you may get some prejudiced opinions. In his excellent book "Can't Go Won't Go: An Alternative Approach to School Refusal", Mike Fortune-Wood says that there appears to be a widespread myth among education professionals that leaving school to be home educated will cause a withdrawn teen to become even more isolated and unhappy. However, he has been unable to uncover any research whatsoever which indicates this is true. Anecdotal experience suggests just the opposite. So if the EWO is negative, you might ask him or her how many teens they have actually known who have left school for home education, and how things have gone for them. (You will probably get an answer such as "Oh, I don't actually know any, it just stands to reason that..." i.e. "I know nothing and am speculating wildly".)
Here are links to various forums for home education and related issues, including one for school refusal which I have heard is good: www.home-education.org.uk/forums.htm Not all parents on that school refusal forum are home educating but I believe it is fairly pro-HE in its outlook. So you might be able to talk to other people who are in a similar situation or have been in the past.