Yup, get a reservation....
Which you can, of course, make online at
http://www.hulltrains.co.uk at the same time as buying your ticket - one of the few TOC sites that lets you do this (thanks
@Flying Dodo).
If travelling from Hull to, say, Doncaster for a connection to Birmingham, that service could well be on a
https://www.northernrailway.co.uk train without the need to reserve a bike space. Which leads to a nice point:
http://www.hulltrains.co.uk will sell you a ticket for Hull-to-Birmingham but won't let you make a reservation for a bike space on the Doncaster-to-Birmingham leg on
https://www.crosscountrytrains.co.uk because you don't need one for the first leg from Hull-to-Doncaster. Should you buy the two legs separately (and be able, therefore, to reserve a bike space on the Doncaster-to-Birmingham leg), the cost is much much more than buying a through ticket! Isn't the rail ticketing system wonderful? #Brompton