Sevenoaks is a fine, academically selective, well-funded, private school. What it does overall, it does well. The IB is fashionable and has helped the school's reputation, especially internationally. And yes, the A Level question is important. I want to share three examples.
A family we know moved out of NW London for this school and they are very happy with it. They are also very comfortable with the IB, coming from continental Europe.
Another family we know, a British family, love the school but now wish their kids had done A Levels as they themselves did a generation earlier. Their elder child, choosing a STEM subject, simply did not have enough subject knowledge starting his University first year, never caught up, lost heart and dropped out. He then studied a completely different subject which required no prior knowledge. His Sevenoaks IB had given him good grounding in the research and the essay writing his new subject required and, accordingly, he did very well.
The younger sibling was a star at Sevenoaks and became a good friend and a good course mate to our daughter at University. Their degree required significant prior knowledge for a key part of it, as well as research and essay writing skills. She, too, struggled to catch up on her core knowledge, never quite managed it and her academic morale suffered. Half way through, she gave up on the expected top degree class and settled for a respectable finals result.
This clearly does not apply in every case, but it would certainly concern me if I were a prospective parent expecting my kids to eventually join the English University system.