It's more complicated because modern trains don't have guard's vans, although some - particularly the East Coast route between London and Edinburgh - do have something similar. But most trains just have two or three bike spaces.
If you have to get a particular train - perhaps because you have an advance ticket - you can reserve space, although I think you have to phone to do the reservation; you can't do it online. So you could buy your non-refundable ticket and then find no bike spaces left. And if you don't reserve, and there are no bike spaces when the train arrives, and you have an advance ticket, you're a bit stuck.
It's easier if you get on at a terminus. More time to find the bike space (they don't mark it on the platform so it can be a bit of a last-minute panic when the train comes in).
It's usually easier on commuter trains with lots of doors and big vestibules, but you can't use these trains during rush hour.
This all sounds very negative, but I was actually getting so stressed out over it I bought folding bikes instead. DW and I now have two folders each. One folds in seconds but is most suited to short trips; the other rides like a normal bike but takes about ten minutes to fold and put in its bag. Horses for courses, as it were. You can take folding bikes on any train as hand luggage.
The picture I posted on the pictures thread shows my slow-to-fold bike on a Cross-country trains Turbostar (on the Nottingham-Cardiff route). I was lucky enough (early afternoon service) to not need to fold it as there was space available. And I got on at Derby where the train reverses so stops for enough time that I could have folded it if necessary.
Yes, it is complicated!