Well they could roll it out. Besides, I can't see why that would prevent electrification - just because the line is electrified doesn't mean the train has to do the full 250MPH, and I can't see anything like the same frequency of trains running from Paddington to Penzance as Paddington - Cardiff Central or Paddington - BTM.
And TBH unless they re-laid all the tracks beyond Exeter, the train would struggle to get above 100MPH without flying off the rails anyway.
What annoys me most about electrifying as far as BTM is that it means passengers bound from London for Taunton, Devon and Cornwall are probably be doomed to change at Bristol forevermore.