Unfortunately, having gone through the Ofcom regulation checker, even a small forum like this, isn't exempt and I'm required to complete a risk assessment form which needs updating every year. Which I'm doing.
As part of that, two of the questions are:
Can children access the forum (this means anyone under 18) and
Can users form closed groups or send group messages
As a result I agree with the guinea pig forum post you mentioned that it would be better if I set the age limit at age 18 and cancel direct messaging. Clearly group private messaging is seen as a possible way for children to be "groomed". Even though that is 99.9% unlikely on here!
Admin privileges override direct message settings, so it means I can contact members by direct message and they can reply. Alternatively members can contact admin via the Contact Us email.
This doesn't affect this forum particularly as we post on the forum and direct message isn't really used or needed much. There is a report button if a problem arises.
Moderators will need to communicate via the private moderator section of the forum (which is rarely used!)
I'll also have to ask everyone not to post their location in future.
I'll do a separate post about it all.