[Closed] Export advice - help

Can anyone help me with figuring out the rules for Exporting stuff ?
I'm lost and confused Googling it.

I plan on filling a container with all sorts of stuff and sending it to Greece (as a business). As a business I believe I can create an invoice for it all, so it can be values at the other end. But I don't know what I have to do about declaring where it originated from.

Some of the stuff I reckon is UK, some abroad and some will have bits from all over. Some stuff I've no idea of its origin.

Can I self declare, do I need to prove any of it.

I'm happy to pay a professional for advice.