This is my understanding of 'expert witnesses'.... Please correct me if I am wrong.
Yes, any independent therapist can become an 'expert witness' because they are 'experts' in their own right. However an 'expert witness' is under oath that their duty is to the Tribunal, ie not to the person who paid their bill. This includes the fact that even though you pay their fees, you are not allowed to change anything in their reports unless they have made a factual error.
All expert witness' reports MUST end with a 'Statement of Truth' to this effect. The Tribunal (government??) lay out the exact wording that the experts MUST use in their Statement of Truth. In my layman's eyes, it reads similar to 'I will tell the truth, the whole truth, and nothing but the truth'. Of course, all independent experts tell the truth (I hope!), but they don't have to sign a legally binding statement to say exactly that.
All expert witness have to follow a fixed format in their reports including their personal CV, list of all documents/people they've consulted, their actual report, quantified & specified therapy (not sure if this one last is a MUST but it's certainly appears to be always done) and a signed Statement of Truth.
From looking at some of the CVs, I can see that many of them have had specialist training from their own governing bodies in being an 'expert witness'. From this, I assume (but could be wrong) that a therapist/EP/etc cannot call themselves an 'expert witness' without doing 'something' (But I'm not sure what that 'something' is - extra training? Licenced by the Tribunal/government? I really don't know.)
This makes it very very hard for any party to dispute an expert witness' findings/reports - from either the parents or the LA.
Most expert witnesses are very very experienced in the SEN system. They have been to countless Tribunals and know all the tricks in the book, including being professionally attacked by the Tribunal/LA/parents/whoever. In some cases, they will also act for the LA. So they are totally independent - despite who paid their (very high) fees.
An independent therapist may be fantastic at therapy but may not stand up well to the onslaught at Tribunals. The private therapy DS has had has been absolutely the top but they themselves admit that they wouldn't cope at Tribunal.
It depends what you want for your outcome. Because of my DC particular circumstances, diagnosis and LA case, he would not have a chance without 'expert witnesses'.