I would very much like HSTs to replace XC's Voyagers. I'd certainly be more inclined to use XC services for longer distances from the South, instead of going via London!
A 5+2 HST would have about 310 standard class seats plus 24 first class seats which would be a 30% increase over a 221.
Even with just 10 sets, that would allow:
- 10 services run by 221's to be run by 5+2 HST's
- 10 services run by 221's to be run by a pair of 220's
- 20 services run by 220's to be run by 221's
However, if XC could have a total of about 17 "new" sets (either 2+5 HST'S or 221's from ICWC) then all their 220's could always run in pairs (it's about because some 22x's currently run as pairs). However with more then even a few 221's could be run in pairs, and once you get to about 29 "new" sets (although, due to spares and some edges of the network not needing pairs all the time, it could be slightly less), all passenger services run by 22x's could always run in pairs, with the HST's having about 335 seats, and pairs of Voyagers having 400, 450 or 500 seats. Which is quite a marked increase over 200 or 250 for most services.
A 5+2 HST should be able to keep to 22x timings (or at least close enough).