Apologies - I appear to have written an essay! But I care passionately about dyslexia!
The problem with dyslexia is that it impacts all education across all the subjects, not just English. This is particularly true as a child grows older and moves through the key stages and then onto secondary school. Every subject has its own "language" with its own literacy skills.
Over 30 years after the event, I can still remember the problems I had when I was in the first year at my grammar school. During science lessons I could not write up the results of experiments - I didn't have the basic literacy skills to be able construct the "language of science". In fact it was my inability to do this very basic function - when I could very obviously do the scientific experiments - which finally alerted my teachers to the fact that something was not right and ended with my own diagnosis of severe dyslexia back in the Dark Ages of the 1970s.
With my own DS, I paid for over 5 hours a week 1:1 with a qualified LSA when he was at his indie mainstream Ofsted crap outstanding school. When I took him out of that school and home ed'ed him, I paid for 4 hours a week with two post-grad qualified and highly experienced dyslexia teachers. This was split between two sessions - maths and literacy.
This 4 hours was still not enough. I had firm evidence that this support had only just pulled his "below average" maths ability to skimming the "average" level but his literacy scores had stayed exactly the same as the year previously when he was in school. In other words, the dyslexia support for his numeracy had helped considerably but the dyslexia support for his literacy had achieved nothing (no progress in one year = nothing) and he was still 4 years behind with this gap yearly increasing.
My DS has great strengths in maths - but his literacy pulls him down. At his previous school, he was streamed into the bottom maths set with all the children who really couldn't do maths and taught by a teacher who was newly qualified and very inexperienced (as is the way with this particular school - the weakest pupils are streamed to be with the weakest teachers). At DS's new indie dyslexia school there is no streaming but his maths is an obvious strength. However his CT has told me that his literacy is such a problem that when he does GCSEs (or whatever has replaced them in 6-7 years time!) even in maths - his strongest subject - he will need a scribe in the exams.
At Tribunal, the LA (mere minutes before the hearing started) agreed to 1 hour a day literacy support with an untrained teacher, but I wanted 1 hour a day with a teacher with a post-grad qualification in dyslexia. The Judge agreed with me - my DS's literacy problems are so severe that he needs teaching with a properly qualified/experienced teacher and that the 3 day dyslexia course run by the LA the HT (of the LA school) went on in the days before Tribunal was no-where near good enough.
As Nigel says up-thread - you can only ask for what your evidence advises. And my evidence showed that 4 hours a week with post-grad dyslexia qualified teachers who both had years and years of experience was not enough. It was on the evidence of total failure on my DS's past educational history that swayed the Judge into ordering the LA to place him in an dyslexia specialist school.
bjk I think you have to weigh up the disadvantages about her boarding compared to her being in a school specifically for dyslexia. My DS has yet to start boarding (long story - could be years away before he boards and it take 1½ hour each way to get to school!) but it's been worth it because the good that the school has done my DS is the very short term he's been there.