I don't know if research in warwich enjoy high reputation in your area, but they do enjoy good reputation in general, and it is diffucult to get the admission from them. So, congratulations! But obviously it is not as good as Cam and OX. So you can just tell warwick that you would like to delay your acceptance for a couple of weeks and you don't need to be too ashamed as this is normal, and the universities are used to such issues