There is in the T&Cs of Advance tickets though. The first one being that, if a train is cancelled, the holders of AP tickets have to wait for the next one of the same operator, unless special arrangements are made. These special arrangements, or the additional wait, would not apply to people with the any permitted ticket. Someone on an any permitted ticket can just hop on the next train.
Where that operator can't get someone to their destination by reasonable means then only then do other operators have to assist. There's a perfectly good RRB, which counts as a reasonable arrangement to get you to your destination, you just don't want to use it; that's not the TOC's issue, to be quite frank.
http://www.nationalrail.co.uk/times_fares/ticket_types/46546.aspx
Here's what they say,
about delays whilst travelling:
If delays occur while travelling, you will be allowed to take the next available train(s) to complete your journey.
And if the train's cancelled before you travel:
If the train you purchased a ticket for is cancelled or is delayed and you still decide to travel, special arrangements will be made to accommodate you on another train (although a seat cannot be guaranteed).
In neither instance do they actually mention anything about the operator of the train.
Where is it stated that additional rights are offered for other ticket types? These terms sound much like what I'd expect on any ticket - if the train's cancelled en route I can take the next available train, and if it's cancelled before I travel, I can choose between a refund and waiting for "special arrangements" to be made.
Granted, if I've got an unrestricted ticket and my intended train is simply cancelled I can get on the next one. But not if it's been cancelled because the line is down - leading to a bus replacement being implemented. In which case I can't just "hop on". I will have to wait for "special arrangements" of some kind.
My point being that it's not clearly stated that I have fewer options for getting around unforeseen incidents if I buy and advance versus unrestricted. This in response to the comments that this limited flexibility during disruption is a significant part of the reason why that ticket is discounted.
In any case, I've not even being arguing about what - having read all small print and T&Cs - my rights are technically. My question is about what seems reasonable, in the scenario I described: where it's possible to get to a destination, using existing train services instead of a RRB.
You're right that I don't want to use the "perfectly good" RRB - I've explained why.
I think that it should be considered reasonable for someone to avoid using a RRB by taking a longer, more time consuming diversionary route by train, if they want to. I think the way our rail system is set up should be changed if the current arrangements preclude this happening. You may of course disagree.