We recently flew LHR-EDI on a Virgin Atlantic flight that was actually an Aer Lingus flight. The flight left from a VS gate with VS GA, but the FA seemed to be a combination of VS and EI, interestingly enough (my guess, could be wrong about that).

My husband and sons had roll-aboards and school-type backpacks (daypacks). No one cared when they boarded the flight - neither the GA nor any of the FA. Not exactly the same situation, but similar.

I had the same concerns and had read the VS hand luggage requirements prior to departure.