Why am I ineligible to return my phone which I received yesterday?

I received a new iPhone yesterday which I purchased from the UK Apple website but I want to return it. The Apple website says I have 14 days from when I receive the phone in which I can return it, but on my Orders page when I click on 'check return eligibility' it says I'm no longer eligible. I should be able to return a product which is within it's 14-day guarantee period and I can't understand why it says I'm not eligible.

I tried to chat with someone online to try find out why, but it just resulted with me back where I started on my Orders page saying I'm ineligible.

Can someone tell me why it's saying I'm not eligible and how I can return it?