As 40% of European air traffic is to/from the UK it's too important a market to disregard and arrangements will need to be made. How about pilot's licenses ? Back to the old British Licence perhaps ?
What about pilots working for Ryanair in EU bases with a British passport, will they need to apply for work permits ? An Irish passport is now much more valuable than a British one as the Irish have always had the right to live and work in the UK and can also do the same in any EU country.