This is precisely my situation. I hold dual US / UK citizenship. When I applied for an ETA using my US passport there *was* an option to select United Kingdom under the list of other nationalities. My ETA is valid until 7th January 2027. My US passport has "United Kingdom" as the place of birth, and I have been questioned by Border Force regarding my citizenship when I was once entering the UK and the eGates were unavailable. I was told that I should use my UK passport as it then means I am exempt from immigration control as a UK citizen. I was also, bizarrely, once asked how long I planned on staying in the UK when I presented my UK passport at the border at LHR. My main reason for using my US passport on short trips to the UK is simply because it works the eGates whereas my UK one does not and I am always told to "Seek Assistance" whenever I try and use it. It is ludicrous that I am able to enter the UK more efficiently using my US passport than my UK one. I doubt very much whether my existing ETA will be cancelled (I guess I will find out tomorrow when the new rules come into effect) and that I will be able to continue to use it until it expires. I always carry both passports so like you TangoOneSeven I'll either be admitted as a US citizen with an ETA or as a UK Citizen with a British passport.