The statement that the UK will accept EASA certificates for up to 2 years now appears
here under the second and third questions. In answer to Q #2 we have "EASA has stated previously that it would accept third country applications from UK Approved Training Organisations. Organisations would need to decide whether, on the basis that the UK’s membership of the EASA system ceases at the end of the transition period and if there is no mutual recognition of safety certificates between the UK and European systems, they would wish to retain both a national and an EASA approval.
The CAA intends to continue to recognise current (and valid) EASA certificates for an initial period of up to two years, but no decision has been made about ongoing validity after this period." and to Q #3 "Yes. your training will be recognised for up to two years after UK participation in EASA ceases if you are training with at an ATO or DTO located outside the UK which had their Approval Certificate issued prior to the end of the transition period and which continued to be valid during the training period.". They also now say, as Alrosa says above, that an "EU issued Part-FCL" licence holder will need a UK CAA validation to fly a G reg aircraft.
The actual rule seems to come from a statutory instrument SI 2019/645, Schedule 3, para 2: "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."