Muhammad ‘Abd Al-Muttalib Al-Houni has hit up one something that most people don’t think about in the whole debate about whether Shari’a should be allowed in the UK (or anywhere else in the West) or not:
The concept of citizenship in Europe will change. There will be [different] classes of citizenship and of citizens, with some citizens being exempt from having the general law applied to them because they belong to a particular religion or belief. There will be a Muslim [class of] citizen, a Christian [class of] citizen, a Buddhist [class of] citizen, a Confucian [class of] citizen, and so on. Each will apply his own laws… Thus, faith will not be an individual freedom or belief; it will [come to] have extremely serious public ramifications…Recognizing all, or [even] some, of these laws would take European societies back to the age before the Enlightenment and the Universal Declaration of Human Rights. As a result, the West would revert to barbarism.
Before the Enlightenment, people were first and foremost "tagged" (for want of an easier word) as being part of a group, not as individuals. In Europe, for example, the group that most conspicuously fit into this category were the Jews. They were governed by their own religious community authorities and these authorities in turn answered to the crown. During the Enlightenment, these authorities lost their civil status either through action of the crown or through revolution, depending upon the country.
On the other side of the Mediterranean, the Ottoman Empire ran the millet system, where various groups (usually Christian, but also the Druze and others) were subject to the same kinds of restrictions. (An illustration of that is here.) The various groups were sealed entities (legally and socially) within the empire. It’s interesting to note that, in the Ottoman Empire, shari’a was still considered to be the supreme law of the land, something that most modern jihadis would find find inconsistent with the existence of millets.
In any case, the concept of individual equality under a uniform legal system is a hallmark of states which are heirs to the Enlightenment. That’s truer in the United States than anywhere else, the Enlightenment heir par excellence. But here the last forty years have seen the de facto creation of all kinds of protected groups based on race, gender, sexual orientation and the like, which has effectively undermined the whole concept. The long term problem with this, as I have pointed out on this site in many different contexts, is that in a society where shared values are as important as they are in the U.S., the existence of protected groups will lead in the long run to persecuted ones, which remains the greatest long-term challenge Evangelical Christians face here.
Be that as it may, I really don’t think that most people really want a society where your entire existence is marked out by the group that you’re tagged to be a part of, at least from a secular legal standpoint. The call for shari’a is the ultimate "calling of the bluff’ of multiculturalism. It would be a supreme irony of history that so many of the "liberating" moves of liberal multiculturalism would lead to the introduction of one of the most restrictive jurisprudences on the planet.