Diagnosis/support for very young children

Good morning, 

I was wondering whether anyone had any experience of having a young child assessed?  I hadn't thought about it previously but during my sons assessment, having my daughter assessed was mentioned. Their Dad also got his diagnosis last year.

I hadn't considered that her development might be delayed because she seemed to be reaching her developmental targets at the 9-12 month review. She is now 20 months old and she hasn't learnt to say any words yet. When she had her last developmental review she would say the 'mamama', 'dadada' or 'nananana' but she doesn't do that any more. Now she just does this repetitive little babble or very enthusiastically shouts 'DAT'. 

She doesn't respond to her name. She doesn't appear to understand any kind of instructions like 'come here'. She spins repetitively until she is so dizzy she can no longer stand or wanders about on her tip toes. I think the worst thing is that she is not able to communicate when something is wrong or let me know what she wants even by pointing etc, so she will often have complete meltdowns and be very hard to calm.

The psychologist who assessed my son said that they would be able to assess her if they had a referral because of her dad and brother having a diagnosis. I have spoken to the health visitor who said that she can see there is a lot of red flags but there is nothing they can do because they can't refer until she is older. 

I have looked for support in developing her speech and language but it encourages getting them to look at you so that they can mimic your mouth movements. She does not like looking at people, she will turn away or bury her head into me and I don't want to force her to do something that makes her uncomfortable. 

She is the complete antithesis of how my son was as a toddler. He was almost overly affectionate towards absolutely anyone he met and would pull people's head towards him to get them to focus on him. He picked up language very quickly and has pretty much talked non stop from as soon as he could utter his first word. So, this is all very new and different for me and I am not exactly sure what I should be doing or if there is any kind of support for children her age?