Wizz will not be hit that hard as just look at Norway and Switzerland, they have to allow movement of people to be part of the single market. Anybody who thinks the UK will be any different are mistaken. There is the unlikely case where the UK won't sign up.
Not so such FR domestics would be impacted and they are also a UK registered company so Im sure they could also pick up AOC quickly.
The UK will no longer exist soon after if England vote to leave!