Apologies, I worded that a bit strongly (yes I’d just rolled in from the pub), but it’s not the same as being in breach of a mobile phone contract (eg I’d be interested to know what happens if you don’t successfully pass out or get sacked one year in). The way it was historically done by TOCs it was essentially unenforceable. However I agree that, if you’re now being made to sign a separate training agreement alongside your contract, perhaps they’ve wised up.
There is more variety with freight work albeit that comes with less predictability/worse shifts (there are exceptions eg Freightliner Intermodal is quite TOC like with regularly diagrammed container trains from what’s been said on here). The arrangement at some is that you’ll know which days you’re working, but won’t know what you’re doing (ie earlies, lates, nights) until the week before.
Those who do it seem to love it, especially enthusiasts who enjoy manual coupling, playing around with locos etc. As you say it’s horses for courses really.