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One example would be Deptford - New Cross. This is charged as Z1-2 assuming travel via London Bridge. There is no alternative route shown, even though you could travel via Charlton and Lewisham (although most people would just walk it!)
Trainsplit use exactly the same wording as LNER though!
https://trainsplit.com/split-tickets
"Unlike other retailers, Trainsplit don't charge a booking fee"
I don't think it is quite that simple, particularly if you are passing through intermediate gatelines. Let's say you are travelling from Caledonian Road & Barnsbury to Harlesden. The only fare listed is £2.10/£1.90 assuming zones 2-3.
If you try and travel via Euston, then when you touch out at...
The problem is people may try and edit e-tickets by changing the date, knowing the barcode won't be scanned. That's a lot easier than trying to change the date on an orange card ticket.