I just got the official email from NHS track and trace, it says that the NHS COVID app is only compatible with devices running Android 6+, if you are running Android 5 it is not compatible. If that doesn't make Google and other OEMs update their devices, nothing will - it is a global danger. 

To check for system updates, navigate to Settings app, then scroll down to about device and check for updates or search for how to update your device, giving the name of your device (Sansung, LG, Sony...)

Stay safe