I take my bike on the train an awful lot and would make a couple of observations;
1. No matter what the official line is, if you stay civilised and polite, the guard is far more likely to allow you to take your bike on the train even if you haven't reserved. The only exception to this is East Midlands Trains, who really do get stroppy if you try and take your bike on a SHF-NRW or SHF-STP service without a reservation, even if the cycle spaces are empty. On a recent (bikeless) journey on a Meridian I even heard the guard make the announcement "As there are no cycles reserved on this service, please note the cycle space will be locked out of use for the duration of this journey"!
2. Why do people insist on sitting or standing in the cycle space, even when there are plenty of other available seats on the train? This is a particular issue on Northern 142 and 150 units, and especially on TPE 185s, where the cycle area has fold down seats.
In general however, I would say that taking your bike on a train is much better than it used to be 15-20 years ago, when you were charged £3 for each single journey and you couldn't relax becuase you never knew if the guard's van was going to be locked and unattended when you reached your destination.