The 70 mph limit is an anachronism. It may have 'right' when it was introduced in the 50s, but in this day and age where the rubber on which our cars run is so much more advanced, anti-lock brakes, disc-brakes and so on, I would argue that 70mph as the legal limit is too low.
Equally, in heavy traffic or bad weather, 70mph could be too high.
So let's introduce a bit of common sense. Where the conditions are busy, say at rush hour, the 70mph limit remains, but where the roads are clear, why not have an 80 or 90 limit ? (personally, having driven in Germany a lot, I wouldn't have a limit at all - but I know and understand we are a nation that likes to be nannied and therefore to soothe those of the '70 is the limit and anything over should result in instant death' brigade, we can still have a limit, but a sensible one).