That is sad, and I feel sorry for the OP.
I am surprised to hear someone would actually want to 'steal' / take an Omega, and never heard of that in recent years!
As he said, it must be for another reason, like the alloy wheels: were they non-standard, expensive and suitable for another car? That is why I do not fit anything non-standard to my Omega that is worth more than the rest of the car and is attractive to thieves. Were wheel locking bolts fitted, if so where was the socket for them kept?
The rest of the car, as he said, is virtually worthless to other people, just a liability really. So has it been abandoned locally; a drive around may find it, albeit up on blocks! At least you would have the car back. Cannot envisage anyone driving it any distance, especially as the guy just approached you - on the spur of the moment or planned? If the later he may well be a local. As already said, worth checking with local scrapyards, as not too many Omega's are available for scrapping now!
As for insurance; for an X reg, not much value there as far as the insurance company is concerned beyond your excess!
Anyway, once more sorry for your loss!