Daysleeper
You appear to be misunderstanding ICAO. The UK and all the other states of the EU are independently members of ICAO, indeed seven of them including the UK are permanent members of the ICAO council. It follows then that anything that EASA does is not in conflict with ICAO and EASA has never sought to be so.
Even if Brexit should happen without agreement there is nothing to prevent the UK continuing as an EASA operator, indeed EASA would like the whole world to join them.