The technology is available. It is already widely used on commercial vehicles, sp, and it is also available for vans and cars:
http://www.speedlimit...l/speed_limiters.html
Not sure of the cost but a retrospective fit would obviously be more expensive than if factory fitted from new. Whether making them mandatory is desirable or not I’m not sure. In general I’m against the State imposing restrictions or conditions that prevent people from breaking the law. (I would not, for example, consider it acceptable to ban the sale of alcohol to prevent drink-driving). But this is, perhaps, slightly different as an alcohol ban would affect non-drivers whereas this would only affect drivers.
I must say I’ve never really understood why anybody would want to spend huge sums on a vehicle that can exceed the speed limit by such a huge margin (and the cost of each extra 10mph capability seems to increase exponentially) unless it is with the intention of breaking the speed limit excessively and regularly. But there’s now’t so queer as folk.