That's where we differ; there's so much electrical stuff in an Omega that I wouldn't use, or needs to be modified so much that starting with other components would be a better solution(heater, instruments etc), that I wouldn't bother removing the body loom from the donor.
Other reasons for this are the sheer size of some of the parts; the fuse boxes and their locations would be a problem in any car I might build.
I have re-wired cars with generic kit-car looms, but the simple ones I used are NLA, and others are at least twice the price. I would be more likely to use a mk3/4 Metro loom now, as it's simple, uses components that I would be using anyway, has remote central locking that is easily used, and is already wired for stand alone EFI. It also uses readily available multi-connectors for parts like lights, column switches, so patch leads are easy to make. Fuse box is about the size of a paperback book, has the OBD connecter, and will fit under the dash near the driver's right knee. Most grounds are made with a multi-plug that bolts to the body work. It's easily removed from the car and cheap which are big advantages.
But we all have different ways of doing jobs this, and wiring is one of the worst jobs of building custom cars.