When the implementation period is agreed EASA will continue to be the licensing authority for the UK. However, the UK would not wish to be beholden to the EU for ever, so it would make sense to re-establish the UK CAA. We have two years and nine months to get the CAA up and running again, which is achievable if the will is there. Whether or not Mike Vivien is up to the job is another question, but apparently 70% of the folk who work for EASA are Brits, so there should be no lack of manpower, if the price is right.