I kept coming back to this question as it would be obvious how much you want help and for some reason not getting it locally where you are.
First, I want to say that I am not a pediatrician and not expert in or experienced in anything but adult dystonias. I do not mind if you ask me to opt out and I do not get paid for this--just wanted to try and help a bit. Your daughter needs to go to a pediatric geneticist and developmental specialists.
There are many, many inherited pediatric disorders that are difficult to sort out and require gene testing to get right. Some of them unfortunately have no known treatment but some have dietary treatment can make an enormous difference like adrenoleukodystrophy, helped in some cases with Lorenzo's oil (named after a boy with the disorder who was helped with it. Some forms of adrenoleukodystrophy have dystonia with it and girls have a different variant than boys. Adrenoleukodystophy is a gangliosidosis--there are many more, some prevalent in people of Ashkenazic Jewish ancestry.
The disorders with dystonia most often have a problem in the basal ganglia of the brain. The basal ganglia work on the control of voluntary movements and are involved with adult disorders like Parkinson's.
In children like your daughter, it is not rare for them to have developmental regression--like her losing most of her language skills.
There is no way anyone could sort this out on the Web. She needs multiple studies and imaging. I just imagine how hard it is to watch your precious child slide backward and also suffer.
Again, if you wish me to opt out I will. If you were in the US I'd know where to send you but there must be study centers in Britain. While I DO NOT think your child fits with Tay-Sachs disease (one of the inherited disorders mostly in Jewish ancestry children) the Tay-Sachs people in the NHS might be people who would direct you properly as to a place that will help. http://www.nhs.uk/conditions/tay-sachs-disease/Pages/Introduction.aspx
I wish you all the best, sincerely.