Before the Severn Bridge became free, wasn't there an era when the DfT» allegedly reduced the number of services stopping at Severn Tunnel Junction on the basis people were using it as a Park & Ride and abstracting from the bridge's tolls?
Quite possible but folklaw usually tints the spectacles and fuzzes and simplifies reality.
I would suspect that with Bristol Parkway as a Park and Ride, the powers that be (whoever they were in those days) didn't want to add yet another station call on the London expresses ... perhaps in addition to the Severn Bridge toll issue. Similar to Corsham with "you want WHAT to stop - are you joking!!"