UK ETA for dual citizen - sh.itjust.works

Anyone had experience as a Canada/UK dual citizen applying for an ETA using their Canadian passport? I’m travelling to the UK from Canada later this year; already booked my flights using my Canadian passport (my UK one expired some time ago). I’ve tried both the UK ETA app and the UK ETA website, and in both cases there is no option to select UK as the dual nationality after I have entered all my Canadian info. Seems like every other country in the world is listed but not UK. It says all other nationalities must be declared, so I’m not inclined to skip over this step. I’ve been emailing with the website support team on and off all afternoon, and they just keep telling me to select “United Kingdom” even though I’ve sent them screenshots of the dropdown menu showing that it’s not listed. Anyone had a similar problem, and have a solution?