Hi there, my daughter just did IGCSEs in Biology and Geography this year. All we did was download the syllabus, buy a relevant textbook, and study it. I contacted about 60 private schools to ask if they would take DD1 as a private candidate and 3 said yes (a low percentage but you only need one!).
We are planning on doing Physics next year and I am hoping my DD2, currently 10, will also be able to take it.
But we have hit two possible issues you should be aware of.
First is that, although 3 schools out of the 60 in this area (by area I mean within a 50 mile radius) agreed to take DD1 at 14, I cannot find anyone who will agree to take DD2. Every single school has said it will raise child protection issues for them and to contact them again when she is older. I'm not going to give up, but you might have extra difficulty getting a private centre to agree.
Secondly, IGCSEs are hard. Much, much harder than a science GCSE. We used lots of A level websites for the Biology revision because the GCSE sites like bitesize didn't come close in terms of depth of understanding. I've heard other people say they are closer to AS level and I can quite believe it - I guess this is why they are well thought of.
Also, there were 3 long exams, all very tight to time. My DD1 is a really quick, really fluent writer, and she had terrible trouble getting finished in the allowed timescale. Thirdly, she knew it all factually, but you need more than that. Lots of the questions relied on applying your knowledge to unknown or hypothetical situations - it was quite a test of logical and lateral thinking. Just knowing it wasn't enough. It definitely needed an emotional maturity that I suspect my DD2, bright though she is, may not have yet.
I'm not trying to put you off - children can and do achieve them young - but it isn't without pitfalls. I hope this is helpful.