"DSs attend an academically selective private school and I am 99% certain they don't do early entry."
Exactly.
One of the things I get really exercised about is the constant whining that private school pupils are disproportionately likely to get into selective universities, even if you control for parental income, education and support (anecdotally, this problem is actually worse in RG universities, where it's a dirty secret, as compared to Oxbridge, where it's at least out in the open). The reason is often, so far as I can make out, that private school pupils take the right number of GCSEs and A Levels in the right subjects at the right time, while too many pupils in the state system do too many of the wrong subjects at the wrong time. A privately educated applicant will have a row of eight or ten core academic subjects at A and A*, with the GCSEs taken at the end of Y11 in one block (often linear) and the A2s taken at the end of Y13. No retakes, at least one MFL, everything either on the "generally acceptable" or "facilitating" list, no complex dance of "equivalent qualifications", no confusion between Music (fine) and Music Tech (not so fine), etc, etc.
In the 1970s, I went to a rough-ish comp and then a rough-as-you-like technical college. But my O Levels and A Levels are exactly the same subjects, from the same boards, probably at the same grades, taken at the same time, as had I gone to the most exotic of private schools. Had my parents spent money they didn't have, I'd have had a bit of a polish but, on paper at least, I'd be no different. That's absolutely not true today, and parents with academic children at state schools need to look very carefully at whether the exams their children are taking are actually the right ones.