Reverserbucket
Maybe the management of the non-UK AOC organisation you refer to should (re)read the EU
Notice to Stakeholders
Certificates issued before the withdrawal date by the competent authorities of the United Kingdom on the basis of the provisions of the Basic Regulation and its implementing rules will no longer be valid as of the withdrawal date in the EU. This concerns in particular:
Pilot licences, pilot medical certificates, certificates for pilot training organisations, certificates for aero-medical centres, certificates for flight simulation training devices, certificates for persons responsible for providing flight training, flight simulation training or assessing pilots' skill, and certificates for aero medical examiners, issued pursuant to Article 7 of the Basic Regulation;
I would think that "anyone otherwise suitably qualified and currently holding a UK issued Part-FCL Licence would (not) be disadvantaged in any way whatsoever for now" might not be the case in less than 200 days from now, following the 23.00 hrs UTC, 29 March 2019 "hard Brexit" "crash-out" that now looks to be the only option of the May Government.