Not that it matters, you don’t have to be in the EU to be in EASA.
Currently the EU28 States are full voting EASA members. Also EFTA/EEA States, Iceland, Norway and Liechtenstein plus independent State, Switzerland are non-voting EASA members.
Unless UK remains in the EEA by rejoining EFTA after 29 March 2019 and transferring its EU EEA membership to EFTA EEA membership and then becoming a non-voting EASA member, UK will become a "Third Country" and, hence, not an EASA member.
The
EASA map shows current member States (use EASA member State filter).