Starling[1] and Monzo[2] have their own APIs accessible to end users which are not Open Banking[3]. Monzo has a separate set of OB APIs and AFAIK Starling doesn't have an OB API as only the big, high-street UK banks (HSBC, Barclays, RBS, Santander, Bank of Ireland, Allied Irish Bank, Danske, Lloyds, Nationwide) were forced to implement the OB spec.
[1]: https://developer.starlingbank.com/
[3]: https://openbanking.atlassian.net/wiki/spaces/DZ/pages/16385...