They would be ideal for Cardiff-Portsmouth to get rid of the 158's.
There are some line speed differentials in the Avon Valley, but would probably be offset by faster acceleration on other sections, the 158s don't race up the hills to Warminster.
If looking for a home for nearly 30 of them them then could add Cardiff- Plymouth (currently split at Exeter), the Devon banks wouldn't be a problem. If it frees up some IETs that can't be bad.
AFAIUI the new Transpennine fleet won't be ready for a few years - it's certainly not going to get EMR out of their current rolling stock predicament.
But it will be same problem if they went on GWR (GBR-GW soon) routes, extra stock is desperately needed now, not in early 2030s when Transpennine can release it.
In both cases, ought to be new stock to replace 35 year old diesels, so might not be needed.
I guess Lord Hendy's rolling stock strategy will have some sort of cascade when it is published later this year. They know when new electric trains (assuming they eventually get ordered) can take over, freeing up 185s