That having been said, Bombardier did originally design the Class 220/221/222 trains to be DEMU trains, which had they been able to add a pantograph car would I think been a lot cheaper answer to IEP hybrid.
I've crunched the numbers on those, and there simply aren't enough of them. Even if you could hybridise them, there just aren't enough sets to cover the whole of IEP bi-modal.
Assuming firstly that hybridisation is even possible; assuming
Nym is correct about needing two pan & tran cars for sets longer than five; assuming that you include the West Coast Super Voyagers and the Meridians as well as the XC fleet
and assuming it's possible to form sets comprised of a mixture 221 and 222 vehicles (this is a lot of assumption!) You could form:
25x 7-car sets, which you make up to 9-car with 2 additional P&T trailers.
80x 4-car sets, which you make up to 5-car with 1 additional P&T trailer.
(That's still a procurement for 130 new carriages)
Now, you need to provide traction for:
- All CrossCountry diagrams, except those that use Class 170 Turbostars.
- All MML Intercity diagrams that aren't fully electrified.
- All GWML Intercity diagrams that aren't fully electrified.
- All ECML Intercity diagrams that aren't fully electrified.
- All WCML Intercity diagrams that aren't fully electrified.
(I'm asuming you'd procure new EMUs for the fully-electrified services.)
You'll end up either procuring new DMUs, or "Chilternising" lots of HST rakes. Neither option is particulary desirable.