The catalog mirrors that of the show's DVD releases, including the recently-returned serials The Enemy of the World and The Web of Fear. Deeper cuts, like serials with missing episodes and the recently-reconstructed The Power of the Daleks aren't included.
But, as a paean to hardcore fans, BritBox has pledged to add the audio versions of those wiped episodes in the near future. In addition, the catalog will also include short trips and side steps, such as K9 and Company, An Adventure in Space and Time and the first, non-broadcast "pilot" version of An Unearthly Child.
The BBC has always been protective over putting the classic show on streaming sites, since Doctor Who is still a big money maker. Hulu previously had a couple of serials from the classic show, as did Netflix, but both libraries were pulled when the license expired in February 2016. Ever since, Amazon Video has held some of the rights, but speculation was rife that the BBC was looking to offer one of its crown jewels exclusively on its own streaming platform.
The fact that the revenue from DVD sales must be drying up means that the BBC now feels comfortable releasing it onto BritBox. Considering that each DVD would set you back around $10, and there are around 130 DVDs to buy, the $6.99 per month rental charge is a bit of a bargain.