You cannot use HMRC’s online services to tell them you’re leaving the UK. Instead, you need to:
- send your tax return by post
- use commercial software
- get help from a professional, such as an accountant
I'm confused, because I found another document that claims that I actually can do this online: Get your Income Tax right if you're leaving the UK .
Claim tax relief or a tax refund if you've left the UK by using the online form service or P85 postal form.
And then it points to this interactive guide.
Which one is it? Can I let them know I left the UK online, or do I need to fill the form, print it and send it via international post?