Why use a HST?
Wouldn't the following options be better?
Modify several Mark 4 coaches with a DVT and a Class 88 or equivalent locomotive?
or
Once IEPs are in traffic, there's going to be a surplus of Class 390s, one could be reformed and refurbished to Royal Train standards and converted for (diesel, 25kV and 3rd rail modes).
My thought was the same, once the IEPs are out there, there'll be a surplus of HSTs as well - I wasn't aware of the IEPs replacing the Class 390s, I thought it was just the Great Western and the East Coast lines that were taking them on for the time being.
I also thought the HST as it is a pretty iconic train for this country, and, as I've said already, is already cleared for use on more lines than not where other stock might not be. For instance, a HST can be used on the Newquay branch; could a MkIV/DVT/Class 88? Could a Class 390? Could those be used down to Exeter, let alone beyond all the way to Newquay? That said, I'm not sure of the route clearance of the existing Royal Train to know or not.
(Once again, I'll reiterate that this isn't something I think *should* happen, as I agree that there's probably little point, it was just a question that popped into my head as to whether it *could* happen, ie, was theoretically possible for it!)