I don't do biology so I can't say much about that subject but for chemistry, I would love to do a proper comparison between the syllabi of different exam boards and examinations one day, but as far as I can tell from a cursory scroll-through on a typical A Level and the IB syllabus, there's at least a 90% overlap when compared with the Core IB topics. There might be differences in the applications that are stressed on but they're mostly the same thing.
However, a large portion of what's included in the IB Options is completely absent from the A Level syllabus. I took A Levels so it's easier for me to tell what's in IB but not in A Level than the other way round.