See the attached screenshots, no trade in £45.79/month, with trade in £36.83/monthIt won't reduce the monthly payments. The value of your trade in will be paid to the lender (PayPal or Barclays). That will reduce the amount you owe them and you will either pay if off earlier, or have less to pay when you come to settle it in the future. You send the old phone to Apple once you have the new one. If they assess the value of your trade in to be lower, they will just pay the lender less, or return the phone to you.
I've now seen the £215 instant credit part so assume if apple value it less my monthly payments would get adjusted up or there's a one of fee back to apple and the monthly payments stay the same