I'm not all that sure that over 100mph, even accounting for the state of modern cars, is safe.
The one thing that hasn't really changed at all in road safety is, to coin a cheesy phrase, the nut behind the wheel: for all that braking distances and crash resistance have improved hugely, human reaction times won't have, nor will the all-too-human tendencies to drive too closely to the car in front, drive at inappropriate speeds for the circumstances, or our generally very poor perception of risk (either overestimating or underestimating risk). So I'd expect to see 100mph running being restricted to only the most ideal roads, under the most ideal conditions, only for modern cars in an ideal state of repair, and driven by drivers who are at peak capability, by virtue of skill, experience, training, and not having been up too late the night before/be remonstrating with a back seat full of kids/trying to find out which junction to take to get onto the M42. By my rough calculations, that probably leaves about 3 cars in the UK
If a variable speed limit system were to work, it'd have to be very closely tied to local weather and traffic conditions, in order that we didn't end up with a situation where an inappropriately high speed limit then led to accidents - people will always tend to see limits as "the speed I can drive at", rather than "the maximum possible speed it could be safe to drive at under ideal circumstances".
I think we're already running up against this issue with the current regime - speed limit signs seem to be proliferating everywhere, with drivers' speeds being micromanaged in 10mph increments (see my post elsewhere), and enforced without any discretion or tolerance. It's asking too much of any motorist to expect them to be doing the complex task of navigating a vehicle through traffic, figuring out routes, etc.,
and having to slavishly maintain their speed to some kind of +/- 5mph target.