Lots of vehicles already on the road with speed limiters.
The minibus I occasionally drive for school is limited to 62mph. HGVs are limited to 56.
There are benefits beyond safety issues too. Traffic will flow so much better on Motorways and dual-carriageways and will handle lane reductions better because speeds are in a smalle range.
The legislation isn't for that sort of speed limiter.
It's more about things like getting cars to change speed if they spot a speed limit sign, but you can over ride it anyway (for example).