I suppose it may turn on whether the CAA retain any authority over the sovereignty of our airspace
This is the whole point - the CAA will not retain any authority 'over the sovereignty of our airspace' or any other part of flight operations or licensing. Once competency is assumed by EASA, whatever they decide becomes EU law, which supersedes UK law, and is equally binding on all memeber states. Should EASA decide that the IMC rating is to go or that all instructors will be required to hold 'European' licences then that is what will happen, no matter what you, I or the CAA may think about it.