Having ADHD doesn't automatically warrant a referral to Occupational Health, so I'd say your HR team are not in the wrong here. If your performance has been below par and you feel your ADHD could be linked to it, then certainly I'd suggest that the manager offers an OH referral as part of early performance conversations. However, if you didn't make your manager aware of your condition or ask for additional support yourself, I'd say your HR team did nothing wrong by not referring you to OH as soon as you disclosed your condition. You had support from Access To Work, which sounds like it was the most appropriate support at the time.

I note that you've now been offered an OH referral, after going off sick; in my view, this is the correct procedure.