Premium Economy on bmi is exactly the same as normal economy, with two important differences:

1) The ticket is fully flexible, so they can change their minds and move to ANY other LBA-LHR flight.
2) They get access to the businesslounge at LBA before their flight.

PE on Virgin is easily the best in the sky atm because its just gone through a refit programme. Lots of info on the VS site.

Make sure they credit the frequent flyer miles somewhere, PE usually yields lots of miles!