How to mirror UV map?

Here is a pickle I am in - in UE4 a model (made by Epic nonetheless) had a normal map baked off the half of the mesh, then the mesh was mirrored (I assume) to make up full model. I didn’t really know that until I exported the model into Blender, fixed it up and optimized it for what I needed it, re-exported it back into UE4.

Now I have half of the model looking as it suppose to look, but the other half look wrong (dark). If I remove normal map, it looks fine, but with normal map lighting is totally off on the presumably mirrored half.

I am at work right now, but this is what it essentially looks like:

I need to mirror UV map and Magic UV addon somehow doesn’t work (I watched a video on YouTube showing exactly how to fix the issue, but while it worked in the video, it doesn’t work in Blender 2.8)

Any help is appreciated.

P.S. No, I can’t remodel hi-poly version and re-bake full non-mirrored normal map.

I´m not sure if I got the idea, but, if you cut the model in the middle, and use Mirror modifier, couldn´t work for you? And, this UV is not unwrapped correctly, but it´s just an example, right?

Nah, that would fix it. I found a solution (I think) for UE4 material, which fixed that mirrored normal map. I hope either Blender gets a feature or someone makes working add-on to flip normals in the UV editor.

Yeah, that’s just an example and not actual model.

To flip an UV island, just select it and use scale in X or Y, using -1 as valor. It will flip the UV.

Right, but I need to flip it on Z, not X or Y… Like, invert the UV island. This is exactly what I am looking for, but somehow it doesn’t work when I repeat steps from the video: