Closing my HSA after moving back to the UK. Do I pay state tax?

Hi all,

I recently moved back to the UK from the US. I am trying to close my HSA account. I am aware if I withdraw the funds for a non-medical expense I need to pay tax on it and a 20% penalty. However, since I am in the UK I am not sure how I should my the state tax (I assume the federal tax is exempt due to the UK-US tax treaty, where I would declare it in a self assessment form here). Anyone has a similar experience who could shed some light on how to approach it please?