If we leave EASA it will become (to UK) a Foreign Aviation Authority, which EASA would either have to:
- cancel thousands of pilot and engineer licences and company approvals....or
- leave them as normal 'foreigners' who have reached all the expected standards and qualified for that authority's licence. Renewals of EASA Approvals/Licences would be the next step...
This should happen under EASA's own Human Rights Laws for the livelihood of thousands of businesses and personnel working under EASA approvals in UK and abroad - worldwide.
If Brexit actually happens, and we become the isolated Island State again, the CAA can only put in place a plan to accept the status quo and convert all existing approvals and licences over a number of years (mainly due to their lack of capability now).
Operationally, things are slightly different as EASA may retract some travel freedoms within the EU but they cannot retract ICAO rules of access and freedoms of flight. This may give Ryanair/EZY and the like, headaches as they would have to be majority(%) EU-owned to continue with all EU airways freedoms. ATC-wise NATS controls a great deal of the northern EU regions too, so that will need sorting too.
None of the above will happen crossing over a knife-edged date...conversions to Parts 145, M and 66 each took 5-year plans to embody.
The CAA could adopt FAA or EASA Certification standards as their own. It is possible, but unlikely within any real time scale, that the CAA could rebuild its former airworthiness status as a single authority...some form of sliding process would have to be built in over a number of years/decades
Nobody is expecting this to be easy.