Which law determines the penalties of not complying to fill out the Electoral Registration form?

I am an EU national living in the UK and have been sent reminders about registering to appear on the Electoral Register. (I moved into m new flat 6 months ago)

I have lived in the UK for 3 years, but so far always managed not to appear on it, mainly because other people living with me registered and just didn't mention me. I now live with my partner, who is an EU national as well. He has been registered before and says he always received lots of party leaflets etc even though he couldn't and didn't want to vote.

I read on the form that it is the law to fill out the registration form and that we may be charged a fine if we don't.

So, first of all, I would like to know which law regulates the penalties and states that it is unlawful not to comply.

Second, it says that if I don't fill in my nationality I will not appear on the ER. Is this a loop hole? Or will they keep harassing me?

Third, the form also asks for my former address. Can I just leave this field blank? What happens if I put in my former address and they find out that I haven't registered before. Will they charge me for not having registered before?

How likely is it that someone will actually come by my flat to force me to register?

Btw. I'm not interested in voting, applying for loans etc. as I don't plan of spending the rest of my life in this country.