iPhone trade in

I am trading in my current iPhone for the new iPhone 14 pro. This won’t be delivered for another 5 weeks but I’m unsure when the pack to send my current phone back will be sent. All I know is I have 14 days to send my current phone back but I don’t know if this is 14 days from purchase or 14 days from when I receive my new phone. Can anyone help please? I’m worried I need to send my phone back within 2 weeks and I will be without a phone for 3 weeks! As I am paying through an apple/Barclays loan I will be charged for my current phone if I don’t send back in time as that price has came off my loan. Any previous experiences with this? (Short back story I spent hours today trying to get the first delivery date for the phone in 1 week and had major problems with my preorder not working so now have to wait 5 weeks!) Thank you.