SFIM, by my reading the original Statutory Instrument says that EASA licences current and valid on exit day are to be treated as if they were issued by the UK CAA, and hence valid for G reg.
SI 219 No 645 Sch 3 para 2
Approvals, licences and certificates
2.—(1) Subject to paragraph 3, any other licence, certificate or approval issued by the European Aviation Safety Agency or by the national competent authority of an EEA state which continues to be in force or effective on or after exit day by virtue of Part 3 of Schedule 8 to the European Union (Withdrawal) Act 2018, is—
(a) to continue to be in force or effective on and after exit day for the remainder of its validity period up to a maximum of 2 years (subject to any earlier suspension or cancellation by the CAA); and
(b) to be treated as if it were issued by the CAA.
(2) This paragraph applies only to documents issued under Regulation (EU) No 2018/1139 and EU implementing Regulations made under it.