I can understand your delema. You have the prospect of quick intervention at significant cost, or waiting for the NHS.
In your position, since the cash is available, I would carefully consider the advantages and disadvantages. I would think about how I could develop, as a person, while waiting. I would think about how I could deal with those around me. I would think about reducing the finincial burdon on my father. He's offered, but by any standard, that is a huge sacrifice for him.
Equally, I would think about getting this thing sorted as soon as I can. I would think about how I could start to build a proper life for myself, my career, dealing with other problems, while I'm still young.
But most of all, I would be thinking of how I can ever thank such a wonderful dad for standing by me in this way. He's doing what all good parents do, being there for his kids. And your dad is a good parent.
If you don't accept his offer, will that make him feel pushed out?
Can you perhaps get your SRS funded by him? The rest you can gradually get in time. Though looking at your photo, you don't really need anything done to that face. (Except, perhaps learning how to fend off the boys!!)