It is in Article 4 of the 'Cover Regulation' to EASA Part-FCL, which is itself Annex I to Regulation (EC) 216/2008 (The Basic Regulation).
Article 4
National pilot licences and certificates
1. Any pilot licence, including any associated ratings, certificates, authorisations and/or qualifications issued or recognised by a Member State in accordance with the JAA requirements and procedures before the entry into force of this Regulation, shall be deemed to have been issued in accordance with this Regulation.
2. By 8 April 2012, holders of national pilot licences, including any associated ratings, certificates, authorisations and/or qualifications issued in accordance with ICAO Annex 1, shall have had their national licences converted into pilot licences, ratings or certificates specified in this Regulation by the competent authority of the Member State that issued the national licence.
It is also interesting to note this in the accompanying Explanatory Note:
The Cover Regulation defines the general applicability of Part-FCL and its other Annexes .... and proposes transition measures for the applicability of Part-FCL. However, it needs to be noted that the definition of a maximum applicability date for the Implementing Rules for pilot licensing in Article 70 of the Basic Regulation has limited not only the periods available for transition, but also the type of possible transition measures. Indeed, since the Basic Regulation establishes that the Implementing Rules for pilot licensing shall be applicable no later than 8 April 2012, any transition measures going beyond that date need to be opt-outs.
Will the UK provincial government seek an opt-out beyond 2012? I wouldn't bet on it.