Disciplinary at work in uk

Just been scheduled a disciplinary at work I work in admin Management feels like there are time gaps in my work where it looks like im not doing anything. I have asd, because I answered the initial meeting truthfully and not sucked up to the boss he decided im not serious so hes scheduled a disciplinary. There was never a discussion about this issue before. I should also add i work through an agency.

Has anyone been at a disciplinary before? Should I tell hr im concerned about where this is going as I might miss certain things due to my asd

I am convinced this is partially because i am not as friendly as the other people in our team. I am also convinced that if they look at other peoples work they will see the same issues.

Anyone got any tips?

Can I just refuse to be in this disciplinary? Is there any way I can have a 121 instead?