im 17 and scared of diagnosis

I suspect your GP can refer you for a diagnosis without your mothers consent. In the UK we have something called Gillick competency, a set of rules that were developed for when doctors can treat people under 16's medical consent as if it was their parents consent. It was originally developed so GPs could give people under 16 contraceptives but applies to other forms of treatment. Your local autism service may or may not have rules about referrals without parental consent but I don't think your GP legally needs parental consent to refer you.

I read that parental consent isn't needed at 16 + but can still be used until 18 . Im worried about any contact of my parents at all. I feel like she'd say that I'm normal and stop me or make fun of me to put me off 

I read that parental consent isn't needed at 16 + but can still be used until 18 . Im worried about any contact of my parents at all. I feel like she'd say that I'm normal and stop me or make fun of me to put me off 

Unless a school nurse can do the same thing as I'm still in education (6th form)

Sounds good to me ,now all I need is confidence and to somehow get an appointment at our underfunded local doctors that supports to many people

doctors are generally allowed to assume children over 16 are competent to consent to treatment. But they can still asses that a particular child under 16 is competent to consent to a specific treatment on a case by case basis. They have to carry out an ethics exercise to determine if you have 'sufficient understanding and maturity to enable them to understand fully what is proposed.' and this is measured against the risk of the procedure which for an autism assessment would be very low.

My understanding of the rules is if they find you to be Gillick competent they are bound to respect your wish for confidentiality unless by doing so you'd be likely to come to harm.