I got one from a UK bank too
Apple has today shared some information in relation to a security issue. This has the potential to allow others to gain access to information on your device if security updates are not up to date.
We are contacting you as you have previously accessed your Starling account from an iOS device. We would strongly recommend that you ensure the security updates on your device are up to date to ensure that your device and information remains secure.
To update: Go to your device Settings > General, then tap Software Update.
If you are in need of further help updating your device, head to Apple Support.