I just came across an issue that I was not aware of and which might be of interest. I was trying to make a booking for a relative who is not in my Household Account. Once I got to the passenger details page I could not enter them as a passenger. I wondered if it was a bug but apparently not...

If you have a Household Account (if not you can just enter the passenger details) and want to make an Avios booking for a person not in your Household Account then you will need to add them as a Family and Friend (go to Manage My Account). You can then select the passenger and complete the booking...

I suspect many on the forum will know this but if not may save you a bit of time...