Originally Posted by
Sandgrounder
I think the wire crossing may come from the right to apply, as British citizen, to bring a non-EEA spouse/family members into the UK via an EEA country. To circumvent our strict rules you can move to another member state, establish residency there by exercising your treaty rights for six months, and then apply for them to join you. Once they have arrived, you can then move to the UK and bring them with you. As they have a residence permit for another member state, the UK is forced to issue a 5-year permit with minimal fuss.
The much more interesting case is endorsed crown dependency passports.
The concept of British Citizenship is a concept far larger than the UK itself.
(Hence far larger than what you quote 'the nationality of a memberstate'.)
The UK is British, other places are also British, that does not make them part of the UK, or the UK part of them. Although they may have an integrated constitutional relationship with each other.
The bearer of an endorsed passport is unquestionably a full British Citizen, but they are not a EU citzen, or rather, they do not enjoy certain entitlements extended to EU citizens.
However when they live in the UK (or Gibraltar, which is fully inside the EU) for a short period, they then as a British Citizens resident in the EU, become EU citizens.
(The fact the endorsements are in Manx/CI passport is irrelevant, it is the endorsements. You could have a Manx/CI passport that is without them and a normal UK passport that does have endorsements, etc).