If you cannot transfer your licence and associated instructor certificate to an EASA Member State before January 1, 2021 then in in order to exercise the instructor privileges in an EASA Member State, or in an ATO located in UK and approved by EASA, after December 31, 2020 you should either:
As of January 1, 2021 licenses and certificates issued by UK will no longer be valid in EASA Member States and will be treated as a third country licences and certificates. Accordingly, as of that date a UK issued ATPL cannot be used to operate aircraft of commercial operators under oversight of EASA Member States.
As of January 1, 2021 in order to obtain a Part-FCL licence from an EASA Member State, UK licence holders must follow a conversion process as per Commission Delegated Regulation (EU) No 2020/723.
After December 31, 2020, holders of Part FCL licence previously issued by the UK CAA cannot be longer considered holders of a Part-FCL licence issued in accordance with Regulation (EU) No 1178/2011 subject to mutual recognition within the EASA Member States. Such licenses become considered as third country licences in the EU after that date.
If a pilot was not able to transfer his/her license to an EASA Member State before January 1, 2021, the pilot still has an option to convert a UK issued license into a Part-FCL licence in accordance with Commission Delegated Regulation (EU) No 2020/723 laying down detailed rules regarding the acceptance of third-country certification of pilots.
In accordance with that Delegated Regulation an EASA Member State may issue Part-FCL licences to applicants who already hold an equivalent licence, rating, privilege or certificate issued in accordance with Annex 1 to the Chicago Convention by a third country, provided that those applicants comply with certain additional requirements and taking account of any credit based on a recommendation from an approved training organisation or a declared training organisation under oversight of an EASA Member State or EASA.