The 70MPH limit was origianlly set up in the 1960s by the then Transport Secretary Barbara Castle, who didn't even have a driving licence. Most importantly, this was a knee-jerk reaction after reports of accidents, and rumopurs of high-speed car tests on the motorways. Plus. the limit was set at a time which &0MPH was probably the top speed of your 1960s car, which back then, would have 1960s drum brakes, 1960s tyres, 1960s steering, 1960s suspension, and 1960s everythng else. perhaps another problem was that people had no trianing/experience of fast-moving traffic back then.
Fast forward to 2006. We no longer drive Ford Anglias in huge numbers. we all commute in the Focus, or the Vauxhall Astra, etc. Most cars seen on the orads are no older than 15yrs old, and they all have disc brakes, some with ABS, they've all got better tyres, better engines, and improved suspension over their 1960s predecessors. They're also much safer should you jhave a crash, too. We may not have motorway driving in thge test, but they're been with us for ages, and some training is available as well.
70MPH may have been sensible for older cars, but tiems have changed. Many parts of the network could safely have an 80, or perhaps 90MPH limits. Besides, if you have a car that isn't comfortable at those speeds, just stay in the left-hand lane, and let the faster traffic pass safely. Simple as that.
There may be conditions where lower speed limits are approprite, perhaps in poor weather, or in roadworks. It's all about common sense really.