Apple rewards visa from Barclaycard was a US thing, so it makes sense. I touched upon it either in the Apple Card thread here on the one on fintechtalk or both.
Apple are partnered with Goldman Sachs for Apple Card. They also have a U.K. presence, and are keen to further expand here too. It would make the most sense to stick with those for the U.K. version too.
Barclays would be the next logical choice, as Apple have had a close partnership with them too over the years, and I recall there used to be an Apple rewards variant of Barclaycard in the US, that rewarded you for spending on iTunes and in Apple Stores.
In the U.K., Apple use Barclays partner finance for this. So it’s a loan, rather than a credit card.