Originally Posted by IO540
non-JAA PPL (i.e. any ICAO PPL, and this description unfortunately includes the old UK CAA PPL) needs validation by the country of the aircraft registration, and they can refuse, or attach entirely arbitrary conditions. In this case, the UK PPL might be more acceptable to a European country than say a Russian PPL, though strictly speaking they both have the same rank.
This may be true outside of the JAA states however EEC directive
91/670/EEC states:
This Directive shall apply to procedures for mutual acceptance of licences issued by Member States to civil aviation cockpit personnel.
Any person holding a private pilot's licence issued by a Member State shall be permitted to fly aircraft registered in another Member State. This recognition shall be limited to the exercise of the privileges of the holder of a private pilot's licence and of associated aircraft ratings under visual flight rules (VFR) by day only in an aircraft certificated for single-pilot operations.
These rules have been in place since 1992, well before the introduction of JAR style licences.