I know Monzo have an API which allows access to your own account (or a few users) -https://docs.monzo.com . It's not Open Banking (although they do support OpenBanking on Plus and Premium accounts - you can view your Monzo account in many other apps and vice versa - I've got nearly all of my accounts showing up in Monzo: including, via my credit report, my mortgage). The "developer API" doesn't show any OpenBanking integrated accounts though so as a "work around aggregator API" it won't be a good option.