Speaking as an N-reg aircraft owner, UK AOPA did not do themselves any favours with a large number of owner pilots by their very ambivalent stance on the N-reg issue over the years.
At one point it got so bad that somebody did a Freedom of Information Act request on the CAA to try to get copies of correspondence, IIRC, between them and AOPA.
I think this may be in the past now, looking at recent UK AOPA statements on the stupidity of the EASA proposals, but it has taken them an awfully long time.
Like it or not, this kind of thing sticks for a long time, and the apparent reason for it (a large number of flying schools, traditionally anti N-reg, are AOPA corporate members) has not changed.
I am a member of US AOPA.