I had tried to bash this rare route in previous years using the 2145 Reading to Birmingham, but in order to see anything out of the windows it needed to be cloudless on the few days around 23 June, and this never happened on the days I was able to go.
For one timetable period maybe in 2018, there was also a GWR train that did this in the reverse direction departing Oxford at around 0400.
So last Sunday I took the opportunity to do this in the daytime.
The late evening train used to be 5 coaches and the driver would walk through. Last Sunday, the XC services were 8 coaches, so the driver could not walk through. There must have been drivers at both ends, which was confirmed when the trains started reversing almost immediately after stopping beyond Foxhall Jn. Not sure if the drivers of the GWR services walked through to do the reversal.
Announcements were made only saying the journey would take "a bit longer than usual" so the sudden reversal surprised some passengers.
Also trains were timetabled to pass each other right in the middle of the west curve - I suppose I could find out whether this was the only way to do it by analysing the timetables more carefully, but I can't be bothered.