I just looked at the TfW “delay repay” website and they quote reasons as train delayed or train cancelled - I’ve selected train cancelled as that’s what I guess it was from my station!
I’ve received delay repay from TfW in the past for not being able to board a full train.
It is a bit of a pain because, as you say, there’s no “full train” option to select, and nowhere in the form to explain either. So I just chose cancelled/delayed, then you get an email in a few days saying it’s been rejected because the train wasn’t cancelled/delayed – but you can appeal the decision, and in *that* form there is a box to explain what happened. My appeal was accepted and the delay repay paid a few days later. I don’t know why they don’t just modify the form, it is a rather silly process – maybe to discourage people?
(I did take a photo on my phone of TfW staff preventing people from boarding due to the overcrowding, and I mentioned that in the appeal, but don’t know whether it made any difference. They didn’t actually ask to see it.)