The reason why they are announcing short Haul rather than long haul is due to the fact that they at the moment just don't have the long haul aircraft to add extra capacity, hence why the B763's and most of the B744's are no longer being retired when the A380 and B787's arrive, yes we will see some fleet retirement but not like for like which was the original plan. BMI is a short haul aircraft airline apart from the A332's which are not being kept, so obviously they want to keep the bmi aircraft utilised to their potential, I reckon over time when the new long haul aircraft arrive or even before they arrive so as to have advertised a new route we will see some short haul freq/routes cancelled in favour of using the slots for new long haul destinations. Only my opinion but I also reckon we will then see the idle aircraft not used as there isn't enough slots for short haul moved to LGW and this will be the fleet replacement.
Remember BA said they will always have to have 2/3 of their routes short haul in order to generate feeder traffic which they depend upon alot.