Regarding science - you are right, you can't do a GCSE science now because there is a practical assessment as part of the exam which has to take place in a school. Schools will not take on private candidates for this as they have obviously not taught you how to do practicals. So that only leaves IGCSEs.
I am a mature student and needed a science GCSE to get a teacher training place. I took on IGCSE Biology as I am not good with Maths. I enrolled on a distance learning course and really applied myself. I knew I was a bit flaky during revision - I just couldn't retain the facts. For me, the two papers I have just sat have been a disaster. IGCSEs in sciences are really for those who intend to take them to A level. Lots of people on the thread for the course (mostly independent schools or selective state) said the exams were ok, but it looks like they are the sort of guys who are going to get 10 A* s.
This is only my experience, loads of others might disagree. I have had to let go of my dream of teaching, despite 6 years of OU study to get a first class Eng LIt degree as I realise no amount of study is going to get me that science GCSE if the only way is an IGCSE. It is possible that you may find it a struggle too, given you also have other subjects to distance learn. Good luck with your studies - it's great you are ready to take them on now.