Not really familiar with that route. The only restrictions I'm aware of is Ledbury and Colwall tunnel on the Cotswold line where there's not enough clearance in the tunnel to open a slam door.
What are the restrictions in these tunnels?
I think a-driver's post covered it. A
HST▸ set running on one power car wouldn't be allowed through the Ledbury or Colwall tunnels. Should there be an engine failure (or worse) you'd want to have the flexibility to get a set out of those tunnels in an emergency. If only one power car is working that wouldn't be possible.
I believe, but am open to correction, that similar emergency procedures are in place for the Severn Tunnel. A HST set, running in passenger service on one power car only, wouldn't be permitted to enter the Severn Tunnel.
The Only Restriction is Ledbury Tunnel, which are no powercars with sliding doors lock out can enter. As if evacuation required that's only way out and no clearance for normal doors.
There are no restrictions on Power Issues on
FGW▸ network except clear runs on certain gradients (Reading Viaduct and Devon Banks for example) Also generally not allowed West of Plymouth on one engine,