Are you sure? The NRCoC say that the Condition 2 or 4 would apply, therefore a brand new ticket should be sold at the full anytime price (as EC obviously don't have a PF policy and Kings Cross has ticket issuing facilities).
Yes, NRCoC doesn't actually say that a new ticket will be sold and is silent on the matter (answer is: it depends)
The EC revenue manual says, under 'C7.1.8. Over-Riding – Travel to a destination beyond that on the ticket', where there is 'Opportunity to buy before boarding' that a full Single or Return should be sold however if cheaper the
difference between the fare paid and the
full fare for the throughout journey can be charged.
If there was no opportunity to buy before boarding, then it is to the
appropriate fare.
In this case, the full fare is much more expensive than the appropriate fare.
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So I'm doing KGX-EDB on standard AP, not stopping at PBO. I do have a KGX-PBO FC season, so was planning on sitting in FC to get the sarnies, then moving as we pass PBO. This is allowed I believe?
This is a completely separate question. The abbreviation 'AP' is best avoided (to avoid confusion) however in this case I can figure out you refer to a ticket of the type
Advance.
I do not think that the rules are clear-cut and an argument could probably be made either way (though I'd be inclined to say that it should be allowed - though you'd forfeit your right to a seat), and my advice would be to approach the guard before departure asking for permission to do this. I wouldn't like to say what the contractual/legal position would be though!