And if that were to happen EasyJet (EU) Ltd would appear overnight based in the cheapest possible backwater of Europe to do exactly the same thing it did 24hrs earlier without restriction. Trade works both ways and so does tit-for-tatting. In the scenario you suggest what makes you think the EU loco's would get an easier ride in the UK than easyJet would in Europe?
The reality would heavily involve the status quo. Don't kid yourself that easyJet is under any big threat in the case of a Brexit. What it might involve is a lot of paperwork and strategy revision. That's work the board could do without but they'll do it if need be and is unlikely to effect long term growth.