I finally went ahead and e-mailed that advocacy organization I found a while back, initially just to make sure they do cover our area because it wasn’t entirely clear.
They say “across the south of England”, but the closest I could find under “choose your local authority” was Essex–and the Essex Advocacy site says they don’t include London boroughs. I also asked if SEAP could point me to anyone else if we’re not covered. So, we’ll see.
Just that was more nerve wracking than it probably should have been. That’s the main potentially helpful thing I have been able to find, and tbh I partly put off even trying to contact them because I’m afraid that they wouldn’t be able or willing to help with what I need. For any of an assortment of reasons.
But, again, we’ll have to see.