If we take the jargon out if it, yes, Starling provides a BaaS (banking as a service) product. And has its own direct connections to things like faster payments (which are referenced in @bee’s link).
They do, though, consume other services which are BaaS-like, if not BaaS. For example, I think they use a third party for payment processing (it was GPS - it might still be).
If you look at Dozens, they provide an aggregated BaaS service - basically orchestrating existing BaaS-like components into something that looks a bit like a bank (Clear Bank, Marqueta…).
So basically these things aren’t quite as clear cut as they sometimes seem.