Leaving UK

I am moving back to the States next year and understand you have to file taxes for the partial year I was in UK.  I have read you need to do this on paper-if so what form do you use and where on the form do you advise HMRC that you are leaving the country. Also when do you file?  I currently do self assessment and understand I cant do a P85 or something similar.  PS ,HMRC helpline has been down for days so any help much appreciated.

I don’t know the answer but it appears that if you normally file self assessment then on your next self assessment include form SA109

https://www.gov.uk/tax-right-retire-abroad-return-to-uk

You'll also find that in the year of leaving you receive a full year's tax allowance irrespective of the date you leave - for most (under PAYE) it can mean a tax refund when filing. Those who leave early in the tax year benefit most.