You can be a citizen (or national) of the United Kindom (of GB and NI) but not simply of Great Britain though.
It also makes it a pain to find, I spent a good amount of time looking through different entries (UK, United Kingdom, British, England, Great Britain etc.).
It also makes it a pain to find, I spent a good amount of time looking through different entries (UK, United Kingdom, British, England, Great Britain etc.).