Hi @Eigen I think the reason that so much is placed on a parents input, is because Autistic traits really do need to be traced back to birth / toddler years.
My mother was initially reticent, but then decided to help. However, if already filled in the gaps if you like because I knew anecdotally that I was picky with food from a young age, painfully shy, puked back bottles of milk and didn't sleep well.
I think, all in all, if you can demonstrate some sort of knowledge from your own childhood, this helps hugely. I'd write a note to go with the paperwork just to say you aren't able to provide anything from a parental pov. I'm sure it doesn't preclude you from diagnosis, as you say, how do people fare who's parents have passed away. It can all be discussed further during the actual assessment.
I also wrote some accompanying notes around milestones. Things like catching a ball - can't remember when I learned, but know I was rubbish. Ditto rubbish at all sport, riding a bike and learning to swim.
If you can, try and provide as much evidence as possible on the things you find you struggle with the most. List them on a separate bit of paper under headings: Social, Communication and Sensory. I feel pretty sure with the amount of examples I gave, I'd have been diagnosed anyway, without my mother's limited input.