Ask professor if she is 'still' willing to write me a LoR? I asked her 2-3 years ago whether she would do that for me, when I was still a BA student, she said yes. I then did not need that letter because I had personal problems, now I am out of uni for some time but would like to apply for grad school. Should I somehow remind her that she once agreed to do that, or ask whether she would 'still' be willing to write me a letter, or will it come across impolite?

I don't want to be impolite, I would just like to somehow remind her that we talked about it previously and maybe remind her a little bit of who I am, since it's been a few years and I cannot show up in person.
There's no reason why you can't ask:

Dear Prof. X:
I was a student of yours for class Y in term Z. While I was still an undergraduate, we had talked about the possibility of you writing a letter of recommendation for me. At the time, I did not end up applying to graduate schools at the time, but will be doing so . Would you be willing to write a letter for me now? I can provide you with an updated résumé/CV.

with appropriate modifications.
You can definitely mention that she previously agreed to write a letter—you might be refreshing her memory, in case she gets a number of requests for such letters!