Oh it's so simple and obvious! Why haven't GWR thought of that? <\sarcasm>
(The "paperwork" no doubt cannot be done by anybody, but somebody with the right competency. Probably somebody who has barely slept all night trying to sort the issues out, rather than "sitting idle")
OK, let’s be more practical here
The Heathrow service is not running fully, so there are spare 387s sitting around, we know 387s work to Swindon to be stabled. We know some GWR drivers sign for 387s, and some sign the route to Bristol Parkway (and some might sign both)
Is there anything preventing a 387 going to Bristol Parkway, (apart possibly from some missing ticks in a table of route approvals), and if it is needs a competent person to approve, I struggle to accept they would be same person who is also a metallurgist surveying welds.
Anyway if the route is approved for 26m electric vehicles with power doors, why would it be unsuitable for 20m electric vehicles (built to C1) with power doors, that draw less power. I have seen plenty of 321s hauled down same line recently.