Unless the UK is a full member of the EU, and not the EFTA that Norway and Switzerland are in, we cannot be "full" members of EASA. Yes, we can stay in (depending on what status of membership is negotiated during the Brexit talks), but if we are not full members of EASA then we will lose all input into the formulation of any regulations that are developed and just have to accept them in whatever form they take and comply with them.