The roads in this country just can't cope with high speeds. Furthermore the skill of drivers don't reflect the suitability to driving at higher speeds. Furthermore there is the ecological reason, it's less environmentally friendly to run the engines at a higher output - more wear on parts, more fuel use. then there's the cost of the increased speed, more petrol, more parts to repair and higher insurance premiums probably too. The roads in the UK just can't take it, they;re too congested. In France, Germany the populations aren't vastly different to the UK's but they have more than twice the area of the UK, therefore emptier roads. On the continent they are very disciplined (well more compared to the UK) about merging, keeping on the inside lane and observations in ratio to the number of drivers in the UK. It's a sad truth I'm afraid.
Furthermore do you really want an 18 year old blasting past you at 100mph in a Saxo?
We'd need more police on the roads to keep an eye out for dangerous driving (which I'm totally in favour of) but again that's a cost, and requires man power and skilled drivers.
Lastly, just because modern cars are safer and can do those speeds it doesn't mean that it's safe to actually do those speeds. I'm no driving god, but I've done a lot more work on my driving (courses and experience as well as track days) to try and get me to a higher level - it doesn't give me a carte blanche to state that I'm allowed and others not, but even with these extra skills I still feel that it's not that safe to blast at silly speeds on the motorway.
Don't get me wrong I love my speed, my adrenaline sports and blasting on my motorbike - but I don't like doing it with other people on the road that I don't trust.