As a matter of interest why should an employer accept a receipt as evidence that an employee travelled on a particular train on a particular route for expenses purposes when a TOC won't accept that same receipt as evidence that the same person travelled on the same trian and journey for Delay Repay purposes?
Incidentially if you hold a number of nicketts for your complete journey so say a season ticket and advance ticket, you need to keep your tickets until you have completed the entire journey otherwise you wont be able to prove part.
So I have recently claimed for delay repay on the home leg (by season ticket) of a journey that commenced in Manchester. The Manchester -London part was on time, so I could have given up my ticket. Fortunately I didn't because the London -Home leg was delayed. Delay repay applied to the whole journey so I needed the Manchester ticket.
Oh and fortunately my employer trusts us to book tickets to where we are going and nowhere else. The outcome is that we can book the tickets on the internal system and print them on rail ticket stock in the office, so have no receipt!
As regards changing employer due to an expense policy, that is obviously easier said than done, particularly as there's nothing to say one won't be jumping out the frying pan in to the fire!