You complete a form at the point of registration to opt out of tax. Is it Medium, or stripe, I don't remember, but I did it. Same as you do when you work for a US client.

Edit: Pretty sure the tax form was for Medium. They sent me the annual summary a few months ago. Look into your settings or ask the help desk how to access the tax forms to opt out, because you’re a UK resident who declares your earnings in the UK.