It was the expert advice that warned me about the odd tax situation. My current understanding is that the UK and US do have a tax treaty and it does mean we don

It was the expert advice that warned me about the odd tax situation. My current understanding is that the UK and US do have a tax treaty and it does mean we don’t have to get double-taxed for this kind of engagement, but to avoid it there is a particular form we have to give to any US-based client that they keep as evidence of the above so they don’t have to deduct US tax at source before paying us. But as you say, if anything like this ever went ahead, this is definitely a taking-professional-advice situation, particularly if that form is at all tricky to fill out.