Many years ago (early 2000s) I was on a Southbound IC225 set that suffered a cracked windscreen on the DVT just north of Peterborough. After limping into the station at reduced speed there was a lengthy delay, presumably whilst control decided how to handle the situation. Eventually the 91 was uncoupled from the rear and attached to the front, where it ran to Kings Cross blunt-end first.
There was talk at one point (mid-life refurb under GNER, when they were renumbered from 910xx to 911xx) of decommissioning the blunt-end cabs, but in the end this didn't happen.
When the 91s were originally ordered there was an idea within BR that they'd be used on freight or sleeper services overnight, which was one of the reasons cabs were provided at both ends. This never happened in the end, but being able to swap ends did come in useful from time to time, as per the incident in my first paragraph.