I am prepared to be corrected but IMHO the slots were awarded to Virgin and if they elect to stop using them the slots revert to IAG so I don't see how EI could continue the EDI LHR service.
My understanding was that the slots were offered up to anyone who wanted to take them on and compete with BA. In the event someone gives them up, they only return to BA if no-one else takes them up to be used under the same conditions. So Aer Lingus should be able to take them if they want.