No, you can’t tell them you are autistic if you aren’t diagnosed. But you could, for example, inform work you have a disability that means you struggle to process verbal information therefore require written instructions rather than verbal instructions, thank you. Then if asked you could provide e.g. a schools IEP (for which a diagnosis isn’t required) with that noted on there or an email chain from college showing the same. The same for other reasonable adjustments that are required due to the disability.
I have nothing to provide. I'm autistic. I can communicate my needs because I know that. For the 40+ years prior though, I wasn't able to ask anyone for anything.
People could also use the same evidence they use to apply for DLA, PIP, blue badge, disabled bus pass, access card… without having a diagnosis.
Can't even get these things with a diagnosis
The Equality Act specifically states the cause of impairment doesn’t need to be established, so they must accept it is possible to prove disability without a diagnosis.
You make it all sound so easy but in the real world it isn't so.