Hebegebes - Apologies, I have only just seen your question.
I think skylark2 is right in that why would you want to encourage an A/A* student, "who may decide later to aim for sciences or even Oxbridge" to take double science in a mixed ability school just for an easy life. The lessons will be pitched at a slower pace and your clever, unmotivated dc may become bored and even less motivated. If he/she then decides to take sciences for A level then there will be some ground to make up by themselves out of lesson time to catch up with the others.
The problem of squeezing triple science into the same time slot as double science is often resolved by starting the syllabus in year 9 but even this is not ideal and there are reports that lack of lab time leaves students less prepared for carrying out experiments at A level. Of course, if the science teaching for triple science is not good in a particular school then it's likely to be just as bad for double science.
skylark2 - presumably though if your dd's very academic private school only offers double science as an option for GCSE then there would be no problem arranging the A level science curriculum around them as the whole cohort would be in the same boat (and I'm guessing that many of the sixth form classes only have 10-15 girls in them). The only problem would be if one of the girls wanted to go elsewhere for sixth form.
I have found this report by Ofsted:
www.gov.uk/government/publications/maintaining-curiosity-a-survey-into-science-education-in-schools
Of interest: Point 68 - expected levels of progress better for separate sciences.
Point 84 - Better teaching took place in upper ability sets (this is worrying for middle ability sets).
Points 100-102 relate to time allocation for triple science.
Also, the stats from the Joint Council for Qualifications:
www.jcq.org.uk/examination-results show that the percentage of A/A* grades at GCSE is much higher for separate sciences than additional (double) science amongst its members.
If you then look at the AS/A2 stats you will see how popular biology is for girls and how unpopular physics is. Indeed biology is often taken as a third/fourth subject alongside English, History, languages etc. which may have influenced some of the replies on this thread.