It should be on your passport too, but I suppose your guardian's passport stamp will suffice to get the correct date.
Trust the student finance website more over anything I tell you (the Home Office does like to make things incredibly confusing so trust them more than me!). If it tells you to select an option because you're not an EU national permanently resident outside of the EU, well... that's you, select that option. I sent you a link on my previous post that will tell you the guidelines on how universities classify students, you'll be able to tell if you will be classed as an international or home student. It's likely that some universities might ask you to fill out an additional questionnaire or ask you to provide certain documentation to classify you.
If you're unsure on how to fill anything on UCAS, seek help in your school.
This is important, and you're not stressing over nothing. Your right to remain and study in this country depends on your immigration status. And these visas and things, they expire, you know? You need to keep an eye on them so you can fix anything and apply for the correct visas in due time. You should be fine for now but please, find out exactly what visa are you/your family on, when does it expire, when to renew it or change it for a better visa...
And finally, do not put you are a UK citizen if you are not one. That is fraud, grounds for cancelling your UCAS application and the withdrawal of any offers you might have received, and potentially more (governments really don't like people who lie about their nationality). Please speak with your family, school, and/or contact the Home Office for help with clarifying your situation.