Travel agents say they are fielding queries about new UK passport rules. Previously, dual citizens have been able to visit the UK on a New Zealand passport, more recently with an ETA, an electronic online declaration costing about $37. "Nowhere does it say you must now apply for a British passport to come to the UK. Thomas said she was sorry dual citizens were finding the new passport rules hard to adjust to. And in fact, you won't even be allowed to board the plane if you don't have your UK passport or your Irish passport.