How do I (adult female) ask my GP for assessment??

You should try to clarify WHY you want to have a diagnosis from an official channel.

it often isn’t helpful for high functioning autistic people 

I'm looking forward to my diagnosis for three main reasons...

• Sometimes when feeling good/fully functional, or reflecting on such times in my past, I start to doubt my self-diagnosis. This leads to me not asserting my needs strongly enough.
• I want to check that I don't have some other condition as well as (or perhaps instead of) autism.
• When seeking medical help (particularly for mental health & therapy), I want to present ALL the relevant information - they more background they have, the more likely to get effective treatment.

I'm looking forward to my diagnosis for three main reasons...

• Sometimes when feeling good/fully functional, or reflecting on such times in my past, I start to doubt my self-diagnosis. This leads to me not asserting my needs strongly enough.
• I want to check that I don't have some other condition as well as (or perhaps instead of) autism.
• When seeking medical help (particularly for mental health & therapy), I want to present ALL the relevant information - they more background they have, the more likely to get effective treatment.

I think these are good reasons and same as mine were. It massively helped understand my MH. I would say, there are days when I doubt myself, even after diagnosis!