Gouli EASA have already stated that any part/installation that has been fitted before the exit date which has a CoA from one of the 27 countries left and is under that NAA's oversite can continue as before and the part still holds certification. Same with any maint checks or work done by UK maint organisations. Anything afterwards is not compliant.
Where the grey area is, is stock. But as most parts are also FAA certified as well there is a process to use that certification to produce a compliance certificate. But that's a by the by, this is just one part of the on going bluff. Even then NAA's may do a France and ignore the EASA edict. Lets face it some of them will be looking at 50%+ of its air carries going out of business anyway. lest face I think there are under 10 mods between a FAA certified A320 and EASA type certificate and they are mostly to do with seat numbers and ozone filters and the like. Nothing heavy structural or engine related.