I made the same mistake today when the Uber iOS app decided to update at exactly the wrong moment and having to hurriedly use their website
Interesting experience - LNER man said “it not possible” then said “try over the road”
A very friendly guy at St Pancras did indeed excess both parts of my split singles for me
Which seems to imply LNER have a policy of … not helping/not excessing/not altering already used tickets/not offering to help people save money … which he was unwilling to breach, but was willing to help me indirectly.
How soon did you realise? With some retailers (including Trainsplit, which the forum's site uses), if you accidentally book a Single and realise promptly that you needed a return, you could have cancelled online and re-booked. Does Uber allow that functionality?
The staff member was probably reluctant because they are not compelled to issue it, and they would see it as time consuming. Saying it is "not possible" is wrong, though some staff take the view that if they don't know how to do it, the default response is to claim it's not possible as it's easier than looking it up.
That said, I am sure some LNER ticket office staff would have done it.