I would have thought the only requirement is that your CPL and IR are JAR-issued, assuming you want to work in Europe.
I believe that although a Spanish issued JAR fATPL is just as valid as a UK one, once you have started in one JAR country's system, you have to complete all the individual components of the fATPL with them, i.e. you can't have a Dutch-issued JAR CPL and an Italian-issued JAR IR.
Anything done outside JAR would more likely than not just count as hour building, meaning there's little point gaining a South Africa CPL, for example.