Before open skies airlines could only operate freely (no need individual agreements), therefore a G- reg airline could only operate flights To/From the UK. An OE- reg airline could only operate flights To/From Austria. Then the European open skies happened, which allowed any EU airline to operate To/From any EU airport to another EU airport. This is why the likes of Ryanair, Easyjet, Norwegian, Wizz, can operate from and to countries they are not registered in.
The fear (note no one actually knows at this point!) is after Brexit that the UK will not be part of the open skies, and therefore:-
Ryanair would only be able to operate to the UK from Ireland
Wizzair would only be able to operate to the UK from Hungary
Norwegian would only be able to operate to the UK from Scandinavia
Easyjet would be able to operate to/from UK to anywhere, but would NOT be able to operate flights not going to/from UK
As all 4 of these airlines have significant capacity either in the UK, or in EZYs case, not in the UK they are preparing for this scenario with new AOCs either in or outside of the UK to retain traffic rights post Brexit.
BA and Virgin are un-affected by this scenario as all of their flights start or finish in the UK
PS. I believe charter flights fall under different traffic right rules compared to scheduled