I have an appointment with the GP

When I went I wrote 8 pages outlining all my "odd" experiences from early childhood, school, college, work etc and what finally caused me to see a GP. This all got sent to a Psychiatrist, who after my 1:45hr consultation said he was 95% sure I had Aspergers (my eldest son officially has it too). Its quite common for it to pass genetically. I'm now waiting for another assessment to verify the Psychiatrist diagnosis. 

I assume the appointment is in relation to self-diagnosis (or suspicion?) of ASD?

If so, you should be taken seriously, but be prepared to provide examples of the issues you are experiencing which have a negative impact on you and you believe may be indicative of ASD.

But more than that, think about why you're asking for help and what you want from starting this process...

Best of luck

well done on taking that first step.

Have you had a look at the AQ-10? It's the first step online questionnaire that many GPS would ask you to complete. I would also recommend writing down things you want to say/description of symptoms/why you want help etc if you find it hard opening up and speaking in person

https://www.nice.org.uk/guidance/cg142/resources/autism-spectrum-quotient-aq10-test-143968 link for AQ10