I have an advance return York - London for 13 July
. . .
It was paid for by work.
You have given quite a lot of information here, but maybe not quite enough!
Let's revisit the NRCoC prohibition on the transfer of tickets:
6. Transferring a ticket to another person
A ticket may only be used by the person who bought that ticket, or the person on whose behalf that ticket was bought. When a ticket is purchased on behalf of an organisation, business or similar entity, then that ticket may be used by any person employed by such entity unless otherwise shown on the ticket by means of a person’s name, photocard number or other identifying mark. In such cases it may only be used by the person so identified.
No purchaser of a ticket may resell or transfer that ticket for value to anyone else unless this has been specifically allowed by the terms and conditions which apply to that ticket, and which will be made clear when you buy your ticket.
It's clear from the above, that a ticket bought by an organisation (perhaps your employer), that it may be used by that organisation's staff in the course of the organisation's business.
In short, an employee intending to travel may pass the ticket to another employee taking that same journey.
But your phrase "It was paid for by work" doesn't quite answer the question: was it bought by the Company, or was your personal expense repaid by the Company?
If the former, then you may transfer it with impunity to another member of staff (it is a right to be conveyed by rail by any person within the Comany or other entity); but if the latter, then it is not clear that the ticket is transferable to other personnel in the organisation, though with an adequate audit trail of evidence, I would feel confident that any challenge in Law to that transfer could be defended successfully, and should indeed succeed.