Angel Trains hold the key to this.
If they were cheaper, no doubt FGW▸ would consider it.
I have seen on another forum from someone who works for FGW in maintainance that even if they charged a ^1 to lease them, they are still much more costly to run then an
HST▸ because they are very labour intensive as they develop a lot of faults. Having said that, as far as I know they have been running pretty well of late.