nasty bit of damage
I just had a look at kind of a schematic of the body parts back there.
I dont know how they could get a "wheel arch" on its own to weld in,
Its hard to describe without a copyrighted pic but:
The piece you would need extends from the middle of the door frame upper and lower,all the way back,including the rear side window and the front part of the rear light mounting aperture for a proper "reskinning".
Looks like a huge job.Would need wax sprayed in afterwards n all sorts.
The sort of thing where a decision would have to be made ,depending on damage as to whether it was an insurance write off or not.
Yours doesnt look that bad.
Me personally,Id go the Vel route
.I got a sideswipe on the front plastic wing and was annoyed enogh about the minor damage
Get a proper repair done and the memory will fade,a welding job ,like Paul says ..
Id allways be worried about accelerated corrosion from behind after a welding job unless it was unavoidable.