Meg 888 your predicament for your lovely daughter is really close to my heart; I have worked in a special needs school for 10 years and mainly with "Post 16". Although completely biased of course, I wouldn't hesitate to send a child who is struggling in mainstream through no fault of her own to such a school as mine. We have no more than 10 students per class with 3 to 5 teaching assistants as well as a teacher. All the students are so very different and their abilities so mixed but it really works them being together. Any higher abilities are recognised as are lower levels ( we have a high achieving 19 year old autistic student working alongside a young lady with profound and multiple learning disabilities and yes they are doing different levels but they both sit in on some lessons together. )
If a student is showing to be progressing really well in a certain subject then "inclusion" can be addressed which will mean he/she can make visits to mainstream school for that specific lesson, with a teaching assistant (a good one will have a radar for bullies and they will not get a look in) with a means to achieving gcse in that particular subject. You just don't get that 1:1 attention in mainstream.
Bullying is so rare in the school; most of the kids have empathy with each other and are so sweet natured it is a lovely place to be.
If gcse is not her thing there are other certificates they achieve such as "Asdan" . Lifeskills is a top priority so on the timetable for our 6th formers is cooking, budgeting, independent travel, college links, enterprise along with numeracy , literacy, pshe, ict, personal presentation, art and music, sports and work experience (lots of it). Our students actually leave school with a food hygiene certificate so are one step ahead of many mainstream school leavers.
If your local special ed school were ours, the head would gladly show you round with your daughter so I am sure others do this.
It is a huge step but I am a firm believer that mum knows best very often. Mum's instinct and all that....meg find a place you both feel comfortable looking around, be it mainstream or special ed and have a look online at the Ofsted report. Even be so bold as to ask parents at the school gate about the school. You care enough about your daughter so go for it!!