It is within the terms of Regulation (EU) 2015/445:
'JAR-compliant training organisations shall be allowed to provide training for a Part-FCL private pilot licence (PPL), for the associated ratings included in the registration and for a light aircraft pilot licence (LAPL) until 8 April 2018 without complying with the provisions of Annex VI and VII, provided that they were registered before 8 April 2015.’;
The Night Rating falls into the category of 'associated rating' if it was included in the RF's registration and
ipso facto a UK RF may provide such training.