I am not sure how 'low cost properly' would work without some facility to check what passengers have with them before they board. 'Check in' leads to costs for staff and infrastructure to enable it. Otherwise it just leads to confrontation on board. I don't really see how it can be done within the fare system easily. It would really need Lumo to sell supplements on their website.Lumo taking from air is just because LNER is too expensive, anyway. Aside from the earlier first northbound they don't really do anything LNER doesn't, they just undercut them by 30 quid or so. They aren't even doing the low cost thing properly i.e. a low base fare with buy ups e.g. seat selection, bigger bags etc.
Similarly, seat booking could be charged extra, but I'm not sure which seats in Lumo's largely unidirectional layout command a premium, and sometimes people sit in alternative seats anyway.
How do you think it would work in a one class train, where one of the principles of the operation is fitting as many seats as possible.