Could you please provide a reference for your statement? Because as correctly stated by turbine100, on 1st January the UK would have left EASA. With no other bilateral agreement in place yet, licence transfers would not be possible.
https://www.easa.europa.eu/the-agenc...and-licensing-
The only reference I find about 31st March is in ORS4 no. 1453 which is unilateral and applies only to those who SOLI’ed out from UK to another EASA member state but awaiting licence issue between 1st January and 31st March.
https://publicapps.caa.co.uk/modalap...etail&id=10016