Cameras are a second best option. The whole problem is "political acceptability", and gutless politicians. As a nation, we seem to express outrage at anything that is seen to take away from an individual's ability to exercise choice - quite why politicians think it is OK for a driver to be able to choose to break the limit is beyond me.
Ideally all vehicles should be fitted with limiters, and then cameras could be done away with. HGVs and PCVs are because, no doubt, its seen as OK to regulate these in the context of commercial activities. Cars could quite easily be fitted with limiters, and the technology isn't beyond us to add GPS so the limiter knows what the speed is. Go a bit further, and bung a basic bit of radar kit could be added in, which could be used with the limiter to tackle tailgating. All of the technology is already there, but we're just too scared to adopt it despite the obvious benefits.
At the very least all vehicles should have a 70mph limiter for both safety and environmental reasons (prevent excessive fuel consumption).