It's... more complicated than that. You can't be awarded a permanent contract as a teacher of a subject you're not registered in. That doesn't stop you teaching it (that's at the discretion of your headie). Likewise if you are teaching it you can have your teaching signed off by your head when you submit you application for registration without any particular fuss being made of how many hours you have under your belt. In this vein a former colleague of mine was part time permanent in their main subject and had rolling temporary contracts for the other subject because they didn't have full registration. Meanwhile I began teaching my second subject without any registration because my employment is solely in my first subject. The GTCS wall is not as solid as many think.