I am dual passport US/UK, residing in the UK.
I converted a 12k traditional IRA to Roth last year when the market was tanking.
My accountant does my US and UK taxes. She logged it on the 1040 4B, IRA distributions and then form 8606 nondeductible IRAs, part 1 and part 2 covers the conversion.
I am coast fire, in the 10% bracket. I owed nothing to the US or the UK on the conversion.