On 29 January 2014 the Preparatory Committee published an "interpretative note" on the consequences of the application of Article 83 of the UPC Agreement. According to this note the aim of the UPC system was not the harmonization of national patent law or for national courts to have jurisdiction regarding the UPC Agreement.
It is the Preparatory Committee’s view that if an application for a European patent, a European patent or a Supplementary Protection Certificate that has been issued for a product protected by a European Patent is opted out (or during the transitional period the case is brought before a national court), the Agreement no longer applies to the application for a European patent, the European patent or the Supplementary Protection Certificate concerned.
As a consequence the competent national court would have to apply the applicable national law.
Link to the interpretative note: http://www.unified-patent-court.org/news/71-interpretative-note-consequences-of-the-application-of-article-83-upca