Yael,
If you are domiciled in the UK, you pay tax in the UK on your world-wide income - including income earned offshore and in the US and not remitted to the UK.
If you are not domiciled in the UK (and most Americans claim US domicile), you pay tax in the UK on your UK-source income only, plus any offshore income and US income remitted to the UK. No remittances, no UK tax.
By declaring to Iowa that you are never returning (in order to break Iowa domicile), you are also declaring this to the UK. You can't say one thing to Iowa and a different thing to the UK. Making such a declaration increases your UK tax by making you UK domiciled.