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?
My wife and children have UK/US dual nationality, so we've run into this. What we do is carry both sets of passports, exit and enter the UK with British passports and exit and enter the United States with US passports. In our experience, the UK authorities don't care as long as you're legal and don't give you grief, and the US authorities will usually be dicks to you for a bit but they do eventually let you through.
That said, we're white. The position might be quite different if you're not.