I have been working as a sole trader for a company based in Switzerland for the last year. This work is fully remote. I have completed my self-assessment and paid all taxes due. The company is now asking me to complete a CA3837 form, but this doesn't seem appropriate as that would imply i have actually been based in the UK. They only want evidence that i have paid my taxes. Have you any advice as to a better way to prove this to them?