Implementing Unreal Engine's rendering features in an external application


I am interested in Unreal Engine’s visualisations potential and already have a project that implemented OGRE, an open source graphic engine, and QT for the GUI and platform compatibility.

Of course, we are now aiming for photorealistic visualisations and are wanting to move from OGRE to Unreal. From the look I had, the way the Unreal Engine is used for most projects is that a project is created within the Unreal Editor and the editor acts as a bridge between Unreal’s features and the product.
Because we do not want to start all of our work again, and that we only need a fraction of Unreal Engine’s features, is it possible to use Unreal outside of the editor? If yes, any suggestions?
For now I am only looking to build a test application outside the Unreal Editor, that would create a GUI with a graphical output that used Unreal Engine’s render functions.

If there is any ambiguity in what I wrote, please ask so that we can try to work out something.
Thank you for your time.