JAR-FCL was adopted by the CAA in July 1999 for aeroplanes. Whilst it was possible to continue to issue lifetime National licences up until 31 December 1999, 5 year JAR-FCL licences were issued between July 1999 and December 1999. All licences issued after 1 Jan 2000 had to be JAA licences and it was not possible to issue a National licence to anyone who had not held one prior to 1 July 1999. Everyone was made aware that all licences had to be maintained in accordance with JAR-FCL from 1 Jan 2000 however; it was not common knowledge that licences issued between July and December were JAA licences and not National licences. By March 2000 only JAA licences were being issued and you had clearly missed the large amount of information that was being circulated by the CAA and the various aviation magazines.
You can convert your licence to an EASA licence and now if you wish obtain a UK National licence as well.