They're externally illuminated reflectorised boards, for the most part.
Here's an example on the Bakerloo just outside Charing Cross northbound. The picture is a screenshot of Video125's Driver's Eye View video. To film the video, Video125 used additional lighting so that you can see more, but you can see the lamp hanging over the speed board.
As someone has already asked - is there any particular reason why one would be used over another? A rule book entry, etc?
I suspect when the specific sign was installed would likely play a part.
For example, externally illuminated signs went out of fashion some time ago, nowadays standard practice is reflective signs. However things don't tend to be changed retrospectively unless something else is occurring in the area. Same with T boards - current policy is to indicate the actual permitted speed, but there are still plenty of T boards to be found. In summary, things are rather inconsistent from place to place let alone line to line. Drivers line and route knowledge comes into play accordingly.