As others have said, some train companies have different rules and even some stations will be more obtuse than others.
I recently travelled by East Coast to Manchester with a bike and the train driver couldn’t have been more helpful, the nod, wink and “nice bike matey! I’ll keep an eye on her for you” certainly helped.
Southeastern can be quite good, however even during the hours of 7pm + that you’re allowed to take your bike on the train, idiot passengers insist in congregating at the bike bays and refuse to allow you to park your bike there (simply because the adjacent seats have more leg room). I’ve had two stand up rows with other passengers, the first bloke got told to shut up when he complained to a guard, the second was a bloke with an expensive Trek road bike who decided that my expensive mountain bike wouldn’t fit next to his (it would). Better signage is needed.
Oh, and First Great Eastern are a bunch of obstructive jobsworths. I once told a guard there that I sincerely hoped that he went home to discover someone else in bed with his wife.