The banner is available still but if you've updated to 5.1 September update you enroll and nothing happens. No beta update is pushed. 👍 looks like you need to be on previous August update. Messy even by Samsung's standards.
I'll be honest. Your not missing much. I left the beta program as the performance was just an awful experience. Crappy animations and gestures. Stick to 5.1 until the stable release is my advice.
No, only phones brought direct from Samsung are eligible to partake in the beta program. Any phones from a network provider I'm afraid you won't be eligible. It's not just a O2 decision.