Surely one fleet, with first class (declassified on StanEx), proper tables and with a trolley service for refreshments on the Norwich route would have been the blindingly obvious answer. There is absolutely no need for a full buffet service on a sub 2 hour (in time, 90 minute) train journey.
Critically, if the buffet service can’t be relied upon to be available, there’s no point having it. The first time a passenger boards at Norwich, counting on refreshment from the buffet only to find themselves on a StanEx unit, will be the last time that passenger doesn’t buy a sandwich at the station before boarding.