I don't think that's a good idea as my bike went in a locked area just behind the driver, so they have to know in advance when to open it. Although perhaps not a problem if you are going terminus-to-terminus.
I went to the station day before as I had forgotten to book the bike when getting my ticket. It was free and they just printed the ticket out.. really simple doing it in person, was a total nightmare trying to do online
Actually, now I think the problem was booking a complete journey where I had to change onto a local train that didn't take bike reservations.
So I bought a seat + bike reservation for Euston-Stoke-Euston, and then a separate seat-only ticket for Stoke-destination-Stoke.