Well, Swindon was reputedly chosen by the fall of Brunel's sandwich from a carriage window (although it did happen to land right by a canal junction where the line's gradient profile changes) so it seems possible that GWR HQ might have been at Abingdon or Sapperton. That, of course, depends on whether he still saw Bristol as the ideal destination when Gloucester via a Sapperton tunnel (as the Thames and Svern Canal used to run) would work more effectively.
Don't think there was much doubt that Bristol was the ideal destination - since it was the merchants of Bristol who were among the people paying him to build the railway. The point was to serve the port of Bristol as it tried to fend off competition for Atlantic trade from Liverpool. It's pretty clear from Brunel's own diaries for March and April 1833 when he made his preliminary survey that he looked at two ways to go west of Reading, the route chosen, via the Vale of White Horse and Swindon - which he recommended as the best - or via the Vale of Pewsey.
cle said:Maybe they should plan a new island, as per Cambridge.
Do you actually know Oxford station? There are two choices - flatten the lot and shuffle things over so you can squeeze two island platforms, plus two freight through lines, into the existing space, with a new station building further east on the existing forecourt, or move down the line to Oxpens, where there is more room to work with.