Another update. I made a commitment to really get my shit sorted out this year, hence why I've been putting myself through the cleaners mentally.
I've received a ADHD diagnosis (combined inattentive and hyperactive-impulsive), plus an autism level 1 without intellectual impairment diagnosis, aka AuDHD. Overall it feels important because it validates why I've found so many areas of life bloomin' confusing and difficult over the years. Also my school reports make sense now, they had lots of "if only Gavin would pay attention", "Gavin needs to apply himself", "If he focussed on the work..." comments. As a result I now feel more comfortable in my own skin.
On a practical level it doesn't change much, because I've spent the last few decades manoeuvring my life into a position where I can be as dysfunctional as I wish without it having any impact on other people. I will be going forward for a medication assessment too, as I'm interested in the ad hoc use of ADHD meds to see how they help.
Posting here in case it helps anyone else who's considering this stuff. It's worth noting that ADHD and ASD have very high co-occurence with depression and anxiety, and that features in the assessment. It's important because if you struggle with them, and have a suspicion of neurodivergence too, it could be worth pursuing to get a deeper understanding of how you operate as a person. As the Greeks said, know thyself.