I think it is probably a blast from the past when the super shuttle flights were re-launched many years ago (1980s) and the entire cabin was treated as a "C" class with full catering including a hot meal on evening flights from EDI / GLA, drink and mini bottle of wine, a far cry from what is offered now, but the flights were crewed properly too with 7crew on a 757 IIRC
Usual domestic boarding passes show "M" not "C" on the (former) non shuttle routes.
Access is possible to a lounge but only on business U.K fares or when connecting to or from another flight in a premium cabin with BA or other "oneworld" airlines.