Morethan - I think there's a difference between changing from primary to secondary and from FE / post compulsory to secondary.
I've been teaching 18 years, and in that time have known a few teachers who have made the jump - both ways primary to secondary and secondary to primary. I have known people do this right from the late 90s to mid 2000s. I know there used to be a six week course you could undertake if you switched. I looked at doing it myself, but in the end moved into FE instead. I don't think the course still exists, or at least I've not seen it for years. However, the way most people I know have made the switch has been through supply. They just do supply in their preferred age until they've got enough experience to take on a class.
However, traditionally FE / post compulsory has been different, because you never used to have QTS with a post compulsory PGCE / PCET. Therefore you were not qualified to go into schools (but we were always able to make the move the other way, as I did as have many of my colleagues). But this rule changed around 2012/13. I know I had a student teacher at the time doing a PCET, and initially she couldn't get QTS, but that changed during her course, and she then got a job in a secondary school where she has been teaching ever since. I know it was touch and go at the time, because the rules were changing but the advice was unclear, so she was offered the job, then told she couldn't have it, before they eventually decided she could.
As for subjects - once you've got QTS, you can teach anything! I've always taught subjects I'm not qualified in and as a RS teacher, you should see some of the people that have been forced to teach RS over the years!!!