It should be banned everywhere. It is an outdated, man-made, barbaric ruling system that contradicts with basic human rights. I will never have any connection with any ignorant human being who accepts such laws. It justifies murder if someone converts from Islam along with many other horrible acts.
The point of a democracy is to vote on the subject. A democratic country may vote to enact Sharia law, or not to. It is up to the people. All "moderate" muslim countries with partial/full democracies don't fully enact sharia law and are unlikely to, even with majority muslim rule.
In Britain Sharia legally operates under the arbitration act 1996, providing each party agrees to the arbitration process. Muslim women are often coerced to agree and British law does not protect them in divorce, custody or inheritance disputes.
A democratic society (especially one with a significant Muslim population) may decide to enact some or all of the ideas in Sharia law by democratic means. As long as the will to enact such a law comes from the people, doing so does not contradict democratic ideals.