I think if you only want to teach sixth form, you will have to be in a sixth form or FE college. I can't imagine any secondary school giving an NQT exclusively A-level lessons in science. Most departments are careful about giving all staff an equal teaching load (wrt to a range of year groups and sets). Your colleagues will want those A-level classes too.
Biology is best-staffed of the science disciplines, so it won't be that easy to find the perfect job.
A very typical set-up is to use teachers in their specialisms from Y9 up, and for everyone to muck in with general science in Years 7 and 8. It's also typical to offer one subject for A-level, two for GCSE and all three in KS3. Even if schools say that they offer separate sciences right from Y7, they won't have any qualms about using any science teacher for KS3 lessons (this is welcome slack on the timetabling.).
Don't write off teaching more than Biology. It is fun to do the other subjects. It is also more interesting in the long-term. It also provides additional perspective to your Biology teaching when you know what they have learnt elsewhere. For example, it is important to know what the pupils learn about big topics, such as energy. The best way to know this is to do some broader teaching.
If you are teaching in HE, you can pretty much decide what to teach. You can't do this in schools, especially state schools. You might find it blessed relief to teach Physics and Chemistry as well, especially if you don't agree with the exam board or government agenda, or if you are told to teach something that is not totally true.
Another big culture shock for many teachers is how weak some of the kids are. You have gone through your own schooling in the top sets (and fairly insulated from poor behaviour), you have enjoyed your lessons, you have gone to a good uni and done well, and have surrounded yourself with top brains in your faculty. This is very tricky to handle, but you do learn to, and you get great fulfillment from moving weaker students on. I am very passionate about the critical relevence of science education for weaker students as it provides essential life skills. This might end up your passion too, so don't rule it out. :)