I don't like this thread. It appears to be someone's attempt to justify their blatant disregard for life - sorry, but speed kills. While there could be some good debate about speed limits - opening with "Ok, I was doing 100mph on the M69 in the first lane! Nothing to be proud of though..." doesn't really set a good tone. Although, it does seem as though the OP feels some guilt about what they have done.
"Speed kills" is one of the greatest myths ever put about. Travelling at 100mph on a clear motorway in a modern car is statistically safer than doing 50mph through a town centre.
It's a combination of a driver writing driving cheques that their driving experience can't cash plus an inappropriate speed that kills.
Now, you may think that you're tooling along at 70 mph as that is what your car speedometer is indicating. I've got news for you. You're actually doing 65-67 mph. All speedometers under-read. It's an agreement between the DFT and the SMMT. A speedometer will under-read more if the tyres are brand new as the wheels are turning more slowly for a given final drive speed. It's why a vehicle's speedometer should be re-callibrated if larger diameter wheels are fitted to the vehicles. A good test of this is if you have a satelite navigation device that will show speed. Get on to a level - it must be level - bit of road and drive at an indicated speed on the speedometer. The sat-nav will show the true and very accurate speed. You may well be surprised at the difference.
The Association of Chief Police Officers has a guidline for speeding. That guidline is road speed + 2 mph + 10% of the road speed. So, the next time that you're approaching a camera don't cause an accident by suddenly braking. Just ensure that you aren't doing more than +10%. On the A3 into London there are three 50 mph cameras, one at Malden, and two just past Shannon Corner. I've never passed those cameras at under 55 indicated and I've still got a clean licence. On a motorway you will not be bothered up to and including 79mph. Overhead gantry cameras are generally set at 80, some are a little more.
Finally, I'm glad that the OP stated that they were in lane 1. Always drive on the left unless overtaking or turning right. There is no slow lane/fastlane on a multiple lane road. A multiple carriageway road should be treated as a single carriageway with a multiplicity (I like that word!) of overtaking lanes.
Oh, one more thing. Belting along is fine but if you really knew how much more fuel a vehicle uses at 80 rather than 60 you'd be horrified.