Historically (i.e. 7 -10 years ago) when selective private schools moved over to IGCSE maths and science courses it appears they were chosen because they were seen to be more challenging and rigorous. According to the links below, GCSE science was very 'dumbed down' a few years ago although I expect things have evened out now. I don't think the perceived easiness in obtaining a Grade C in English Language IGCSE has much relevance to IGCSE maths and science.
www.theguardian.com/science/2009/mar/27/gcses-science-dumbed-down
www.independent.co.uk/news/education/schools/the-appliance-of-real-science-should-all-children-take-the-igcse-2026801.html
"Parents at Parkside became even more alarmed when curriculum reform in the mid-2000s, ostensibly to make science more relevant and accessible, brought in a new emphasis on ethical issues, such as organ transplant and abortion. The new "tick-box" multiple-choice exams did not require pupils to write any scientific term or recall formulas, and led to widespread concern. A group of teachers took the unusual step of launching a Downing Street petition deploring the loss of scientific content."
"Even the official exam regulator, Ofqual, called on the exam boards to improve standards, arguing that questions about the advantages and disadvantages of CCTV, mobile phones and the internet were no substitute for the principles of physics."
"Under pressure from scientists and teachers, the Government ordered a report from its curriculum advisers. The report concluded in 2007 that the IGCSE should not be endorsed for the state sector because it was too difficult and was out-of-step with the national curriculum, covering material that had been moved over into the higher AS- level exam."
www.theguardian.com/education/2009/mar/04/private-schools-gcses-manchester-grammarschool
Martin Stephen, high master of St Paul's school in Barnes, west London, said: "The new GCSEs (2009) are appalling for the most able students. They are simply pap, they are baby food, they are examination rusks in too many subjects, and they do not stretch and challenge the most able." The school offers IGCSEs in maths, science and music and is considering expanding them.