I genuinely don't have a problem with the 70mph speed limit on motorways. It's a good, economical speed for getting places. Most of the time I sit at 70, those who want to go a bit slower can do so, and those who want to steam along at 90 do so, realising they're at the mercy of whatever police officer they drive past. If we raise it to 80, what changes? People still drive slowly if they want to conserve fuel or are too scared, and carpet salesmen in white Audis will still tool along at 90+.
Other areas of road design and driver tuition need much more urgent attention, really, such as random speed limit reductions followed by a speed camera. Or speed cameras in general. And traffic 'calming' measures. And teaching pupils a bit more about lane discipline, making decent progress and other practical areas of driving.