Internal vs public facing services
Whether your website or mobile application is used by staff or open to the public it will need to be accessible.
If your application runs in a browser or on a mobile device it will be covered by the Public Sector Bodies Accessibility Regulations 2018.
If your application is a desktop application which is not browser based it will be covered by EN 301 549 - Accessibility requirements for ICT products and services.
Both standards require your application to be accessible and adhere to the Web Content Accessibility Guidelines (WCAG) 2.1).