As a Mobile based company, I feel it is essential to keep your app updated to the latest and greatest and keep it consistent with all platforms wherever possible.
That being said, here are a few suggestions:
Bottom Navigation - As our phones screens are getting bigger and bigger, the introduction of bottom navigation has largely been adopted by many apps on Android, it will make switching pages much more easily and effectively and speaking of consistency will match your iOS based app.
Oreo Support - It would be great if you could update the app to ‘Target’ Oreo API 27, which is the latest api and keep it up to date forth-going. API 27, introduces many features such as, Notification Channels, Memory optimizations, Autofill, Fingerprint improvements, Adaptive icons and many other security, fixes and improvements. Starling Bank on Android is currently targeting Nougat api, which is near 2 years old and with Android P around the corner, you sure wont want to left behind
One thing I like about Monzo, is that their app is kept up to date with the latest apis and features, and feel Starling should be no exception.