A related question if I may. If I book an advanced ticket from Nailsea to Durham and cycle space from Bristol to Durham, what would happen if I am refused boarding on the local service into Bristol because there are already too many bikes? Firstly, I then have missed the advance ticket reservation, but more importantly, what happens if every other XC▸ service that day has all the cycle spaces fully reserved (as may be the case as it will be when universities are returning).
Purely from personal experience, I've come back from BHM with a booked bike and found all cycle spaces occupied. I put my bike in the long luggage space (XC Voyager) on
TM‡'s advice – alongside another bike (don't know if booked or not) which was already there. And similar on other occasions. In practice, no one seems to check bookings on XC. Not done this all the way to Durham though...