As per Chris B's second paragraph, this is the supporting evidence for a full refund if you don't travel.
The Advance Fares internal
FAQs▸ that were doing the rounds a couple of years ago included this:
Q06 - Are refunds or changes allowed with these tickets?
A: Once booked, the ticket is not refundable. Customers will still be entitled to a refund in the event of a delay or cancellation as per conditions 25(a), 42 and 43 of the National Rail Conditions of Carriage.
NRCofC has been renumbered since, and condition 25 (a) is now 26 (a):
if the train you intended to use is cancelled, delayed or your reservation will not be honoured, you decide not to travel and at that time you return the unused ticket to any ticket office, the Train Company responsible for that ticket office will, wherever possible, give you an immediate full refund as shown in Condition 27;
Clearly you must theoretically ask for this 'full refund' at the appropriate time you decide not to travel, I reckon you'd get no joy a few hours later, or the next day...
(There follows various caveats where the ticket has been bought online, or from another
TOC▸ etc etc, but that only affects how the 'full refund' is processed and paid, not the basic entitlement.
Paul