My condolences on the tough lesson. Clearly you will not be playing it so close to the margins in future - we are all busy paying for our education in one way or another.
I agree with your basic theme that it is rough that IG changed their terms mid trade. It should be outside the law under the Unfair Contract Terms Act, but whether it is is another matter - and an incredibly expensive one to establish in court. The reality seems to be that most companies write in clauses to their contracts saying they can do whatever they like.
This may seem like small fry, but do you remember when Air Miles were divided in value by a factor of 10? - which to most individuals amounted to relatively little, but to the perpetrators represented a huge windfall profit. The law on that derived from unilateral contracts related to promotional 'gifts' of plastic rubbish, but the reality is that uk courts seem to allow unilateral changes of terms. And the economic circumstances at that time would obviously be used to justify the action taken to reduce risk.
Other comments on this thread have pointed out that it is unusual for spreadbetting companies to be defended in this forum. Again, that does not augur well for any legal action you may find yourself embroiled in.
Though it goes against the grain, I would take your site down, as the cost and time of running a defamation action are beyond the pale. Also the cost and time of promoting your site to a top position may well be better spent in another direction.
Personally I have reduced the time I spend spreadbetting and increased my systematic trading. Feel free to take a look at investmentadvice-online.com
All the best
Pete