I was in the same boat.
I got my new phone coming on Friday as well. I actually called up Barclays and paid off the full balance. Then booked my phone with rebuy for collection on 20 Nov, after collection I will call Apple to cancel my previous AC+, hopefully a refund will be issued and sent to Barclays, which they should return to me 😀
Is this a thing? Can we get our AC+ refunded?
Also I've looked up the best prices and for my own 11 pro max 256gb unlocked, EE have offered the best price - just checked Rebuy and it wasn't a great price. Then i am hoping to use Carphone Warehouses trade-in promise and beat that by £10 further.