I beg to differ. Don't forget the title of this topic, the class 91s and mark 4 coaches. In my opinion, the NRM should be making plans to preserve both 91031 and 91010 when they are withdrawn from service. The former is worthy of preservation as the last loco built at Crewe works and the latter as the fastest loco ever to run on the UK rail network. I'd like to see 91010 kept in the Battle Of Britain Memorial Flight livery and displayed next to Mallard at York with 91031 returned to Swallow livery with a set of matching mark 4s (DVT+FO+RFB+TSO+TSO+TSOE at minimum) to run occasional railtours (unless a third-party preserves such a set, in which case 91031 could be put on display at the other NRM site, or possibly next to the APT-P in Crewe Heritage Centre).
IC225 may not be quite as iconic as IC125, but in my view deserves to be.
Sadly it looks like most of the 91s are to suffer premature redundancy as the new IEP fleet arrives, but the mark 4 coaches should be cascadable.