It's been a while, but in the Before Times there were a couple of TOC's websites that still used the older API that allowed you to book cycle spaces and disability assistance at the same time as seat reservations. Not sure what the current state of play is...
But ultimately, reservations are only really worth the orange cardboard they're printed on. Other than the few services where you need a member of staff to provide access to the cycle compartment (who may or may not bother checking them), they do exactly nothing to ensure that the space isn't full of one or more of: an unreserved bike, the handlebars of an adjacent reserved bike, luggage, bags of rubbish, pushchairs, standing passengers, crabs. The rest of the time they function mainly as a tie-breaker if you and someone with an unreserved bike try to claim the space at the same time.
It's shit. This is why the TOCs keep congratulating themselves so loudly about improving cycle parking at stations.