A pilot who holds a Part-FCL licence which was issued before the UK left EASA other than by the UK may NOT fly a G-reg aircraft after 31 Dec 2022 unless they also hold a UK-issued Part-FCL licence.
If the non-UK Part-FCL licence was issued after the UK left EASA, the pilot may NOT fly a G-reg aircraft unless they obtain a UK validation or licence conversion.
I apologise for the stupidity of the UK having left EASA!