Motorway speed limits will only ever be reduced – not increased.
Aracer – Not only will every single slip road and junction need altered, every single sign, gantry and safety barrier will need replaced.
The lengths of a slip road is designed to allow safe braking from the time you leave the motorway to the junction at the end (although there are a few instances where this is not the case).
Signs are designed so that you can read them from a given distance while travelling at 70mph, all to do with speed / time / distance travelled / reaction.
Although safety barriers are tested at speeds greater than 70mph, they can't be guaranteed to work if hit at greater speeds. The cost of replacing all of these throughout the country would be in the £billions.