The Bristol - Oxford service was (IMHO▸ ) doomed when First took over Thames Trains; there was little point in them competing with themselves when the judgement was that the majority of passengers would put up with the inconvenience of a Didcot change.
This service ended in May 2003, a year before Thames' franchise ended. And it was a joint
FGW▸ /Thames service anyway. The reason the
SRA» wanted it killed was that they did not want 90mph two-car Turbos pottering about in the way of 125s, when coal trains heading to Didcot power station already made pathing between Wootton Basset and Didcot a nightmare. That and the fact that passenger numbers on the through trains were always low.
If anyone had ever funded reopening of Corsham and Wantage Road stations, then things might have looked a bit different, since no-one would really want to stop 125s at either place but both schemes are still stuck on the drawing board.