Hello StickyFrog,
Thank you for the feedback and I'm sorry about the export issue.
First, make sure that Affinity Photo is the active window and has focus before right-clicking the BMP icon. If Photo is not the focused application, the export command will do nothing.
Second, please check Windows Task Manager to ensure that Affinity Photo and BMP Export are running at the same privilege level. If Photo is running in admin mode and BMP Export in user mode, exporting will not work. The 'Elevated' column in task manager is what you want to verify:

Please let me know your results/findings. I'd like to resolve this issue for you and incorporate updates to the program as needed.
Thanks!
PS-I appreciate the tip about the AV flag. I'll add a note to the homepage for awareness.