@Dblock
I read as much as I could from this topic. I apologize in advance if I missed something from what I'm going to say.
You don't seem to take into account maintenance costs and, more importantly, reliability. While a BMW 330d or 530d might get around 40mpg (though most owners I've come across say it's around 35-38 for the E60 530), they are awfully unreliable. Unless you talk about the latest (F gen) models which should be too new to have issues, and I don't know much about them.
It's the same with the 2.2cdti engine from Honda. I heard some horror stories about the injectors, HP pump, and their oil drinking habits.
Then there's the DPF on modern diesels. If your commute distance is too small and you rarely get to drive them on long distances (though you did mention monthly trips to Glasgow, so you should not have issues) the DPF will clog, overheat and ultimately fail, which will add more to the cost of owning it (a lot more).
By comparison the GS 450h seems to be one of the most reliable luxury saloon ever built. It really doesn't get better than this.
Good luck making the right choice.
