UV-Packer for Unreal Engine is a free, fast and exact tool for automatic packing of UV maps. UV-Packer optimally packs even massive geometries with a single click without the need to leave Unreal Engine or reimport assets. 

The main advantage of UV-Packer is the automatic workflow of rescaling UV clusters relative to the surface they cover on the mesh and arranges them to utilize as much space on the UV map as possible. This increases the texture quality for the mesh, fixes overlapping UV and results in optimized light maps.

For more information about UV-Packer and how to use it you can find the documentation here: https://docs.3d-plugin.com/uv-packer/unrealengine