There's a bug in the way btinternet handles emails sent to the +owner address, or any other +something address.  See https://groups.io/g/GroupManagersForum/topic/34125793#18881 for details.

The messages are routed to the whole group.

When responding to a moderation message, be sure to select just Reply, not Reply All. Or use the web interface for moderation activity.

A resoultion to this problem is expected (allegedly!) in January 2020.

Update: Appears to be fixed 25 February 2020, https://groups.io/g/GroupManagersForum/topic/71532154