Marlespro,
I'm very surprised at your post. On a couple of occasions, I've emailed the embassy to clarify that I could attend, and they said of course. Also, I sent an email to confirm that the interviewing officer can witness the I 864 as I will be attending with my husband, and they said yes, the interviewing officer can sign, so long as I'm present.
I would strongly urge you email the embassy at LondonConsular@state.gov and clarify this issue. I don't know the circumstances of your case, but perhaps when they said "family members" they meant family other than the sponsor. Perhaps there's a misunderstanding, because, if I remember correctly , when you fill in the document checklist for interview readiness, there is a space that asks who will be attending with the sponsored immigrant.
Please check this out. When you email the embassy, they respond pretty quickly.
Good luck and let me know what you find out.