Adverts mixed into departure boards?

[Bletchleyite]

Does that mean a seat can become reserved while you are sitting in it a la Crosscountry?

Yes, most seats are marked with an amber "may be reserved" light. The ones you can sit in without such concerns (only about half a coach worth) get green lights. However LNER make it much easier to get a reservation just before travel (with seat selection) than XC do.