The UK is a third country and ICAO signatory. As a result, pilots from the UK who fly in other ICAO signatory countries are required to comply with ICAO requirements. For sailplane pilots, the ICAO requirement is an SPL with a class 2 medical. The EU and the UK have licence validation requirements.
Pilots who intend to fly outside the UK are advised to comply with any new administrative requirements in good time. Further detail about these and other changes following EU and EASA exit are available here.