Starting in 2026 the UK no longer allows British Citizens to get an ETA to enter the UK on an American Passport. They must use their British Passport to enter the UK.
The problem I’m seeing that I can’t get a straight answer to is by US law any US citizen must enter and leave the US on a US Passport. So with that being the case how is a dual citizen with two passports meant to fly from the US to the UK?
Leave the uk on your uk one and enter the us on your us one.
As long as you’ve got a valid plane ticket and aren’t on the no fly list, the airport doesn’t care about whether or not you have permission to enter whichever country your plane lands in. That’s your problem for you to deal with.