To be fair they were made available on both the operator’s website, and on the booking site the OP used. As I pointed out above, even in the context of consumer contracts, it’s common for the “tick the box to confirm you accept the Ts and Cs” method to be used to incorporate terms into the contract. That, in itself, isn’t something anyone should be surprised by.
Are these restrictions onerous? Well, luggage restrictions in the context of a travel contract are hardly unusual. Okay, hitherto we haven’t normally experienced actually
reaching such limitations in the context of train travel, but the NRCoT itself has limits on what you can carry, so theoretical restrictions are already present.
Unless I’m missing something, there is no “penalty for non-compliance” and certainly no “forfeiture of any right to travel” stated within the Ts and Cs; you are simply advised that certain items of luggage may not be permitted. Therefore the OP theoretically could have discarded the disallowed item and still been permitted to travel.
I bet you a tenner there isn’t

.
If I was the OP, I’d be going down the road of chalking this one up to experience, and perhaps asking for a goodwill payment if I felt like chancing my arm.
I don’t want to be accused of being “anti customer” here, which I’m not, but I am surprised at the level of outrage towards a low cost rail operator who are merely doing what many low cost airlines have been doing for decades.