MIT sent you a letter saying they would have offered you an unconditional, two months after you declined? Somehow, I doubt its as cut and dry as you make it. Either this means they would be willing to reconsider, hence the letter, and there'd be no big mistake at all, or theres something you've not made clear. At any rate, Imperial can and must release you if you ask them to. They don't have to release you to another UCAS choice, but if it was an American university, UCAS would have nothing to do with it anyway.