I haven't read the whole thread so apologies for any duplication etc.
This whole case is going to revolve around wether you knew what you were doing or not.I.G. have a legal and moral obligation to " hold your hand " whilst you are in your early days of trading.This is why they have to ask you, as all equivalent firms do what your experience of trading certain instruments are e.g. options,futures,shares,indicies etc. If they deem you to be inexperienced then they should put you on a limited risk account and be seen not to be " fleecing " you.The fact that you are on a full account makes it seem that you knew what you were doing.One defense that you could use is that you were inexperienced in that type of instrument and they should have realised that.Only you know what your experience level was at that time and even then it wont be easy.
Then there is the question of why I.G. closed the trade when they did.They have traders working through the night,why wait til £90,000 + ? It could have been closed out a lot sooner.
I would fight it.I can't see anyway in which you will get off without losing your deposit and credit limit but try and get it reduced as much as possible.The amount will depend on your experience level.
Good luck.
I would imagine that the FOS has already looked at that, and decided in IGs favour. Their opinion seems to have been that IG acted within its terms and conditions.