You have the offer. Don't worry. There was just a mess up in their email systems which made them think that you have accepted the offer, which you have not. Funnily enough, on the morning of the last open day at Warwick whilst we were driving there, my daughter got an email from Warwick saying that it was sad that she was not able to make it, only to get another email shortly after to say that it was a mistake. Clearly they have a few glitches with their automated email script.