OK, it's not really an 'app', but it behaves a bit like one - add a shortcut to this page to your device's homepage or desktop. You can install it on your phone/tablet/computer and it opens this page - but not in your browser.
PC (Edge or Chrome), Mac (Chrome) & Android phone
Click the 'install' button below. You will be asked if you wish to install the app on your device - agree to this. The current browser window will close, the app will open in its own window. An icon will have been added to your desktop; on a phone, you may need to manually move it from your list of apps to your home screen
iPhone or iPad
Swipe up from the bottom of the screen, then swipe left until you get to 'add to homescreen' select this. The shortcut will be added.