The trouble is that these minis seem impossible to find. In most cases I didn't even know of their existence till long after the episode they are prequelling has aired. Then after discovering their existence there is the challenge of finding a site that steams them. Talking with a group of Whovian friends today I learned there were two prequels I missed at the the start of this (the ninth) season.
The other mini episode was The Doctor's Meditation, I couldn't find a video to embed but at the time of posting it was available on DailyMotion. I then spent some time trying to learn if I missed any more mini episodes. During my searching I found a helpful list of mini episodes on Reddit.
The last thing I learned today was that the BBC has a digital Doctor Who 2015 Advent Calendar.
Countdown Till Christmas Special Airs: 20 sleepsCountdown Till This Whovian sees the Christmas Special: 21 sleeps