Fair point re reporting numbers, but using an X in place of the letter assuming the correct one isn't on the wheel still gives a good indication of what train it was used on. From the context of "I used this ticket on the xxxx train from Nempnett Thrubwell to Portishead on dd-mm-yyyy" it should be pretty easy to deduce the train which 2X34 instead of, say, 2R34, refers to.
And not just because there are very few trains between Nempnett Thrubwell and Portishead