So Apple Pay cash is looking to be US only at launch of iOS 11 but this news at least shows UK and European adoption may not be so far behind!
For me this would be competing with PayM rather than Monzo to Monzo.
The initial launch issue for Monzo will be lack of Apple Pay support. As I believe that is how you will be withdrawing any balance from Pay cash to your desired account.
As iOS 11 is due 12 September (or fast following) so should have a lot more information very soon…!