This may well be anathema to Matelot but one option to stop the spread of extremism (and bad faith based ideas in general) might well be to help develop the nations that produce terrorists.
Punishing the potential criminals in the UK will surely achieve next to nothing in the long term. We need to try and change the way these people think, globally.
There is a well proven correlation between development, reason and stability.
Much of the developed world has progressed to a point where religious groups are little more than social clubs- with ever dwindling memberships.
"Norway, Iceland, Australia, Canada, Sweden, Switzerland, Belgium, Japan, the Netherlands, Denmark, and the United Kingdom are among the least religious societies on earth. According to the United Nations’ Human Development Report (2005) they are also the healthiest, as indicated by life expectancy, adult literacy, per capita income, educational attainment, gender equality, homicide rate, and infant mortality. Insofar as there is a crime problem in Western Europe, it is largely the product of immigration. Seventy percent of the inmates of France’s jails, for instance, are Muslim. The Muslims of Western Europe are generally not atheists. Conversely, the fifty nations now ranked lowest in terms of the United Nations’ human development index are unwaveringly religious."
Harris, Sam. Letter To A Christian Nation (Kindle Locations 477-483). Transworld. Kindle Edition.
The US bucks this trend quite spectacularly. I have no idea why.
As a rule, as countries become more developed they become less religious. Interestingly, countries with low religiosity are also the most charitable "both in terms of the percentage of their wealth they devote to social welfare programs and the percentage they give in aid to the developing world."
Harris, Sam. Letter To A Christian Nation (Kindle Locations 502-503). Transworld. Kindle Edition.
It is surely undeniable that these terrorist attacks are relgiously motivated. If we can help make countries more developed, and make them less religious as a natural consequence, they will attack us less (or not at all) as their primary motive has been removed.
We have to stop this problem at source, not just punish the result.