The premiere UI tool during UE3’s reign was Scaleform. The only way to use Scaleform now is getting an Adobe Creative Cloud subscription for access to Animate.
You do have other options, like drawing your UI directly to the canvas, but you will have to write a lot of very basic stuff yourself. If there’s no support for buttons, for example, then you’ll have to make your own HUD class that keeps track of widgets’ render textures, their locations and depths, and then check the mouse coordinates to see what the mouse is over, and then call a function when that particular widget is clicked on. That’s all premade in Scaleform, and you can access it all with a single addEventListener in ActionScript.