Leaving the UK permanently, P85

[ECF]

Hello everyone! I really hope I can get some help with this matter, as the official departments are very hard to get a hold of and online is not very clear what I should do!

So I am an EU national and I've been living and working here for the past 5 years but in less than a month I'm leaving the UK permanently. I just got my P45 and I was about to send for legalisation my documents, but I found out I have to fill in a P85 form. The problem is that when I try to fill in online, they ask me a past date of my leaving the UK, which I can't obviously. But ideally, I'd like to deal with all documents when I'm here and I don't know how it really works.

So please, if anybody could clarify this matter for me it would be amazing!!

Thank you!