Unlikely to be anything to do with Oxford University (esp as what devon_metro posted isn't actually correct

). "Full term", meaning classes, lectures etc, starts on 11 Oct (for some reason the university week always begins on a Sunday). Most colleges expect the undergrads to arrive by the Thursday before, except the freshers who will have been there a few days before that. There's no one big influx/exodus in practice, and students tend to trickle in steadily over a period of a couple of weeks. And it's not as if they all travel up from Devon either!
Definitely odd that the booking system says there supposedly no reservable seats on the
HSTs▸ between Exeter and Reading though, I really don't understand that one.