the UK CAA have already said they will accept EASA licences for up to 2 years after the transition period. Also, there is nothing stopping EASA license holders from getting a UK licence and holding it alongside their EASA license (again the CAA have said a simple process will be put in place next year).
Not so fast....where is all that stated?
The current working assumptions for the UK CAA's Brexit stance are clearly stated here: