I used to drive at between 75 & 80 mph on motorways in this country, but now tend to do around 65-70 as it's easier on fuel consumption. There's also less risk of being done for speeding if one sits at the lower level too..!!
For me, there are two issues that are inter-connected. One is safety and the other is enforcement.
Whilst modern cars may be deemed as being "safe" (and they generally are much safer than they were say 25 years ago), individual drivers are always an unknown variable. At higher speeds, stopping distances increase markedly and that brings into question the ability of both car & driver to safely stop from higher speeds, should something untoward happen.
The other issue is enforcement. With the current speed limit being 70 mph, many people will tend to do "just over" this, as they know there's not much chance of being caught. Raise the limit to 80 mph (as was proposed a few years back), and people will take that as a green light to drive at around 90 mph. Again, you then come back to the ability of car/driver to stop safely at those sorts of speed.
I'd personally be in favour of increasing the speed limit on motorways to 80 mph, but only on the proviso that this would be enforced more strictly that the present 70 mph limit seems to be - purely due to the issues outlined above.