I guess there could be a
system where some board members are elected by supporting members, and
others by guarantor members?
No, I don't think so, at least not without drastically changing from
the model articles. Directors are elected at an AGM and all members
(which legally speaking means guarantor members) have a vote at AGMs.
If we wanted to change that we would need to get a solicitor to write
it all up properly, I think (at the moment they just need to rubber
stamp it).