I think the simple answer to your question is that it is perfectly possible, but the government has never sought to introduce such a law. If they did, what speed would you set it at?
If you chose, lets say, 80, then on motorways many people would simply drive at that speedall the time.
The trouble with speed limiters, as shown by those fitted to lorries, is that they foster an attitude of wanting to go at the maximum speed without consideration of ather factors.
There is a danger in taking some parts of the decision making process away, in that the mind can relax a little too much.
This has been encountered in aircraft, since aircraft would be perfectly capable of doing it all themselves these days. The problem is in keeping the pilots sufficiently alert to monitor it.