The reason they are retainng 4 slot pairs is for competition reasons, personally I think they will discontinue EI's services to LHR and then sell the remaining 4 slot pairs to BA
In fact, I understand that the reason there are 4 LHR slot pairs excluded from the "agreement" is that these slots don't actually belong to EI - they are leased to EI by VS and are presently used to cover the 3x daily SNN-LHR and one of the daily DUB-LHR routings. As such, the assumption is that they will be returned to VS at some point in time.