Hi wiggy. I would presume they would lease aircraft, just like Monarch's aircraft were leased. They will find a way to get aircraft otherwise why would they be bidding for slots? I am in no doubt that BA/IAG want to take the slots to stop not only further easyJet but also further Norwegian expansion from LGW.
Why would BA need to lease aircraft? I doubt it could get hold of all the Monarch slots but it could get hold of a dozen longhaul 777 hulls very quickly if required to fill the ones it wanted.