Enemy of the World was released on DVD in Britain November 2013 and in North America May 2014. Although, Canadian Best Buy somehow arranged to have a special advance release in December 2013 and was available exclusively there until the general release in May. Web of Fear meanwhile was released on DVD in Britain February 2014 and in North America April 2014. This might be of interest. Basically, it's a list of Doctor Who DVD releases with their release dates in the individual regions listed. We can see that with some they were released same month, while others have many months between. So, really there is no reliable precedent on the matter.