The death of physical media is an exaggeration. There will always be people who cannot, or do not wish to use digital services. Yes, the market is diminishing, but it will not disappear anytime soon, if at all..
There'll always be a niche audience who want physical media, just like there is still a niche an audience for music on Vinyl. But for the vast majority of people streaming and downloads are the future.
Midlist/catalogue releases are rapidly diminishing for DVD and never really took off for Blu-Ray (which is why you find companies like Twilight Time specialising in 3000 copy print runs of cult titles that would once have received normal releases), so we will reach a point in the next few years where the only things sitting on physical shelves for sale will be new releases. (And anything else you'll pay through the nose for.)
Web & Enemy did well thanks to the huge publicity surrounding them but a physical release of TUM would be lucky to hit Moonbase level sales, especially since two of the episodes would be just recons and not even animated.
Like I said, I'd be amazed if they don't show up on the BBC Store when it launches in the near future.