I was diagnosed with autism through the NHS as an adult a few years ago and I get quite a significant amount of support through mental health services, council housing and disability benefits. Getting the services was more closely linked to my MH issues than the autism though - I was receiving the support before my autism diagnosis. The therapy I've had hasn't been that helpful (although I've had a lot of sessions) because the therapists had no understanding of ASD. I've had a significant number of suicide attempts and I self harm and have an eating disorder, so I can definitely believe the statistics.
There are support groups locally but I haven't attended them, but as far as I can tell they are open to self-diagnosed adults so there is no need for a diagnosis if your BIL wanted to attend. I'm also a member of various online groups but again, you don't need a formal diagnosis to use those.
I haven't been able to work for years so I don't know much about employee protection, but I've been in education and the diagnosis has helped me get support and adjustments for deadlines etc. Also, I've never had to claim JSA when out of work, I'm on ESA (support group so no interviews) as the work related activity and any back to work schemes would have been massively stressful for me. Financially I'm quite well supported as my rent is low and I get the highest rates of PIP and ESA.
So having the autism diagnosis is very important to me, I feel more comfortable asking for help about it than I would about my MH issues, and it helps me understand my problems much better.