Personally, I would do a parental application. This starts the 20 week timescale and school will be asked for their opinions in the assessment period. There is a model letter on the IPSEA website you can adapt. Include any diagnosis letters/reports and any EP/SALT/OT etc advice/supporting info with your application.
www.ipsea.org.uk/making-a-request-for-an-ehc-needs-assessment
If you get an EHCP it means that your child will get additional support. Specifically for PDA, in our section F (the support section) my child has things like
All staff will be familiar with her trigger points and to avoid anxiety-raising situations likely to stoke anxiety or agitation, such as a count-down to finishing work, or corporate consequences
Teaching staff will receive training on the low arousal approach
When very anxious, strategies for PDA/anxiety-based avoidance will be used, rather than assuming avoidance is a choice.
Staff will use low-arousal techniques.
Staff will give choices.
Staff will use of negotiation.
Adults should give calm measured responses in confrontational situations
There are a number of other recommendations not specifically PDA related that mean that my daughter needs small classes, academically able peer group inc girls
The LA have placed her in independent mainstream school but she could also have got a specialist school with her EHCP.
Your daughter may be OK in mainstream with support but PDA children often fiercely oppose looking different and my daughter would have only coped in mainstream with a full time TA and coming out of class A LOT.
The PDA society have a lot to say about types of schools good for PDA. I have to be honest, the mention of PDA is going to put some schools off, but if it does, perhaps you don't want her there?!
Have you got a school in mind? An EHCP will take about 5 months min and, if you have to appeal for a specific school, could easily take a year so its worth applying now if you want her settled for year 9 and 10.