Agree with many of the points on here - especially PJH's points about everyone not exceeding a 100mph limit by such a margin.
I have been incredibly lucky thus far to never get a speeding ticket (by my own admission, one of the greatest miscarriages of justice in recent years, although I have had a few 'discussions' with the boys in blue who IMHO have handled the situations very well - once got an almighty b*ll*cking by the side of the road when I was 17, along with a couple of friendly chats in later years!) so I do not have an axe to grind in this respect, but I do think that the majority of motorists would like to drive to what they feel the corrects limits for themselves and their car would be.
There will always be some nutter who wants to break the sound barrier whilst swerving across 3 lanes of busy traffic, and I guess many of us have at one point or another gone a fair old bit over the limit (<insert your own number here>!) on rare occasions we deem as safe to do so, however even if there was no limit I would personally not drive at 120mph plus on regular occasions. The reality is that higher speeds are more tiring to drive at, fuel economy can take a serious tumble, and the world does start moving a bit quicker when these higher speeds are reached (slight bends or bumps in the road become somewhat more noticeable at 140mph!). I also agree that schools and residential areas should have low limits that are adherred to (would be nice if common sense was enough to make this happen without enforcement), since this kind of road is far less predictable and the potential for something bad to happen is much higher (as all the stats, for what they're worth, also state).
Even without a limit, I'd probably tend to do 90-100mph on the way home up the M40 on a sunny day (exactly like a lot of the traffic does now!) - some days I'd go slower because it is a more relaxing way to drive. On the way to work I'd probably do exactly the same speed as I do now, because the sheer weight of traffic controls the speeds, not imposed limits. Speeding tickets seem to be enforced as a way of generating revenue (nothing new here!) under the banner of saving us from ourselves - my opinion is that driving is not without risk, therefore we will sadly never get to the point where there are no road accident related deaths. However, I would much rather that the authorities attempted to save peoples lives by targeting those driving dangerously (driving too close, stupid overtaking, ducking in and out of lanes on the motorways just to gain a 1 car advantage, etc, etc) since I genuinely think this would make far more of an impact than going for the easy target of someone doing 85mph on a clear motorway in a car capable of much more than that.
Guess this will always be too tricky and less profitable to police, so whilst there is the easy 'fleece the public' option, I'm not holding my breath for it to improve!!