RJ’s point about incentives
easyJet will need an incentive to operate from SOU just as much as Jet2 will need one from BOH.
In business, it’s always wise to seek more than one quote from a supplier or in this case an airport. I would be very surprised if easyJet were not pitching the two against each other in order to obtain a better deal at SOU. If they aren’t then how do they keep their costs in check?