Most of the UK operators required their employees to have a UK issued licence (at least for flight crew), simply tranfering your licence to an other state is a breach of your contract.
This situation is even more stupid as you could have obtained any rating (TR, instructor certificate) in an other state and issued those ratings on your UK licence before the 31/12. No one knows if those ratings can now be transfered on an EASA licence. It's been almost a month that the UK-EU TCA has been signed and the EASA hasn't published anything for the validation, even though the AIRTRN.18 states that "certificates of competency and licences issued or rendered valid by one Party and still in force shall be recognised as valid by the other Party"....