What we do know is that — similar to Apple’s iOS App Store when submitting an app — the Apple Pay approval process for a new bank partner is not for the faint-hearted. Industry insiders tell me that Google Pay has less hurdles to jump in comparison.
It was also an approval process for a new card processor which is extremely rare. This website, though an old article, indicates that most (all?) existing UK banks outsource card processing (see the section ‘The Processor’). So even the big banks didn’t have to go through the hassle of having Apple certify their processor.