The best way to do this is via constitutional protection that can't be overridden by ordinary law. The EU has a de-facto a constitution in the form of the ECHR

The best way to do this is via constitutional protection that can't be overridden by ordinary law. The EU has a de-facto a constitution in the form of the ECHR and the various treaties, which do protect fundamental rights, but there are so many caveats on those rights that a carefully worded law always risks slipping through.