A validation of a U.K. licence, or at least a quick and easy conversion should be allowed by EASA. Especially from a country that was previously part of EASA. The standards in the U.K. are still a lot higher than in certain EASA countries.
Some non-EASA countries allow operation with a simple validation of an EASA licence, others require a conversion with only a single Air Law exam. EASA can easily apply this for U.K. licences. (Like the U.K. is recognising EASA licences for the next 2 years without any paperwork.)
However, those non-EASA countries recognising EASA licences do this because they are in a need of pilots which can’t be covered by local pilots (e.g. Turkey, U.A.E., Qatar, etc.). So as long as there isn’t a need for non-EASA pilots in the EU, I don’t think this will change soon unfortunately. There are plenty of unemployed EASA pilots available at the moment, so I don’t see EASA accepting non-EASA licences at the moment.