If they hold a current FCL compliant licence issued by any EASA Member State other than the UK (since we're not technically a MS now, but are treated as such) then that licence will be automatically validated on G reg aircraft from January 1st until the first expiry of relevant ratings. Any UK National licence issued at the moment does not allow flight on EASA type aircraft, and in a discouraging oddity is unlikely to next year since the initial Statutory Instrument covering withdrawal with no deal retains the current principles, and will restrict non-FCL licences to flying "non-Part 21 aircraft" with Part 21 aircraft being those that are currently classed as EASA aircraft. Something that could so easily have been done overnight on December 31st.....
I am told from recent meetings that the forms to issue UK-FCL licences for non-UK EASA FCL holders will be live by January 1st. Hopefully. All dependent on the political process of withdrawal of course, since at the moment the CAA do not know what the withdrawal format or allowances will truly be. The "no deal" legal arrangements are presented in SI 2019/645 on the uk.gov site. Only 152 pages.