the old program was offered by Barclays Bank. It allowed you to spread your payments over time, according to a schedule acceptable to Barclays. After you applied and were approved, they gave you something strongly resembling a credit card with [no added interest] payments over time.
IF and ONLY IF you made each and every payment in full and in time, there was no interest added.
If you missed, they charged interest from day one, at typical rates for credit cards.
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The new program does not appear to be a credit card, but more of a line of credit account. Its entry page is here. Examples on this page are all iPhone examples, but ignore that.
iPhone Upgrade Programme - Apple (UK)
Read, then click the (Explore financing offers) to learn details, including offers for Macs and iPads.
it looks like Mac over 6 to 12 months can still be had for 0 percent interest, PROVIDED you never miss a payment.
The terms have some complexity. Be sure to read and understand.
There are also terms for restarting with a new device, by turning in your [working] old device.