While registration is a big issue, ownership has the potential to be an even bigger issue. Both Easyjet and IAG will reconfigure their ownership structure to become majority EU owned. In the case of EasyJet, this is just changing a declaration. In the case of IAG they will either force UK shareholders to sell their shares or offer additional shares to EU shareholders. Both have significant financial implications. The only problem is that currently the law in the UK will mandate a majority UK ownership for domestic flights, as the 'Great Repeal bill' will just transfer the current rules. Unless the government changes the law, which might not be easy, then BA and EasyJet will not be allowed to fly UK domestic routes. If the law is changed then there is the potential for airlines from outside the UK and EU to enter the market. All in all huge change to the status quo.