I follow the SOW, so in terms of what topic I'll teach next, that's already decided. The kids are set, which makes it a lot easier.
What level I take it to is decided by the class. I'm constantly assessing them. I do a bunch of examples, questioning as I go along helps me figure out who's getting it and where the problems are. Then after I think people are getting there, I'll give a question for the class to do in their book independently. Then I'll ask who got it right. Do they want another question as a class or are they happy to do some independent work? Depending on what they say, I'll go through it again, or set them off on an exercise while I then go and help individuals who are still struggling. I set homework, mark it, and then go through bits they all found hard. If they are finding the examples easy, I'll push them harder than the SOW suggests. I've got a stack of textbooks in my classroom, I may well abandon my original worksheet and flick through to find an easier or harder exercise, or make up my own questions on the spot.
Today with my Y9 bottom set we were going to be multiplying decimals. It turned out that they couldn't multiply whole numbers, so we spent the whole lesson on that. Next lesson we might attempt decimals. My lesson planning is quite ad-hoc to be honest, and levels vary from lesson to lesson.
Our baseline assessment isn't levelled, by the way (neither are any of our termly tests until the May exams) so no, we couldn't give that level. When the SATs boycott was happening, we did do levelled baseline assessments (proper SATs paper style) and so we do have a reasonable idea of how much the SATs levels are over-assessed (hothousing etc) by the time they get to secondary.
Re tracking in other schools, as I said, levels are designed to track progress between key stages, and sub levels are made up. So no, I don't think that any school could produce reliable numerical tracking data that is that fine tuned to produce accurate sublevels of progress over the course of a year. And a student producing a piece of work that is levelled at a 6 is not the same thing as saying that that student is a 6 in that subject.