Whether or not your vote would have made a difference is a moveable feast. If, for instance, you are a 'minor' creditor (with less than 25% of the debt compared to those who vote ) it might not have made a difference. But, if, say, you were the only one who actually bothered to vote (or has more than 25% of the debt of those who vote), you could have insisted on amendments or scuppered the whole arrangement ! There is no way, as you were left out, that the IVA provider can say whether you would have made a difference or not as, if rewinding time, they cannot predict who might or might not have voted in a different time stream where you were correctly included ! (All getting a bit Star Trek !! ).
Their offer of compensation is an admission of negligence and, I would imagine, you accepting this would be in 'full and final compensation' and you might then be unable to claim your rightful share of the dividend if you were to be now included --- this might be more than £300, albeit stretched over a 5 or 6 year period.
I am not trained in insolvency, but I would think that the IVA, being based on a lie of ommission, could still be challenged. You would need the advice of a lawyer with experience of insolvency law. I understand, from historical posts, that Lisa Thomas (a forum member who is also and IP) might have a contact here. Hopefully she will comment in due course.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014