Oh, and re: the 'Senator problem' indeed, the separate model was a brilliant idea, not forgetting the Senny, aside from sharing underpinnings and engines, also shared stuff like front doors, which many people can't believe, they are so artfully and skillfully styled, they appear much bigger on the road and totally different cars. Anyone who has seen the front end of a Senny grafted on a Carlton will know how close they are.
But clearly for the magic 'cost' reasons, they decided not to pursue a model up above the Carlton/Omega A. Presumably the marketing decision was to make the new Omega move up in the market place, where the Carlton had previously been that slightly bigger than a Cavalier, a bit Antique dealer/Farmer maybe, a slightly workhorse machine, with a bit of comfort. In 1978 when they launched the Carlton, no-one could have imagined just over ten years later there'd be GSi Carltons, super-luxurious ones with full leather, walnut & air-con and then even Lotus turbocharched ones. So perhaps the logical progression was to move the Omega up a notch, meaning that a proposed Senator replacement would itself have to be moved up a notch, meaning directly in the firing line of Merc S-class, which was something they weren't prepared to do/knew it would never wash with customers. (and anyone who said people would pay £40-50k for a Vauxhall would be proven wrong when the Lotus Carlton did not achieve the sales it was hoped to be - the Ferrari F40
outsold it, for instance!)
Of course that's not to say I don't adore the Senny - which I do - and wouldn't have
loved to have seen a true replacement - which I would. Look at a Holden Statemsman if you want an idea what it would/could have looked like.