For some reason many muslims believe that any criticism is a lie and that the devil is tempting them, they may not say it, but that is their concern. The fear that if they listen to you and they'll doubt even a little bit is the path to Hell. Even though it's really annoying when they do it, i don't bother debating with them anymore unless they start first.
My advice, don't bother debating with them unless they start, then at least theres a chance they'll actually listen to you and you'll ba able to have an intellectual discussion with them.
I think it is even simpler than that - many Muslims make being a Muslim their whole identity; any criticism of Islam is analogous to attacking the person personally.
Then look at where they come from; Pakistan, Bangladesh - they're not real countries, they're fictions, creations, there is no national identity - this is why you find that those countries who export the terrorism tend to be from countries that are fake. All of the Arab countries are merely byproducts of colonialism - the question of what makes an Arab and Arab is still raging. Pakistan and Bangladesh were creations based on single source which that causes problems if you have a multi religious society.
When you identity is derived from a single source - you'll view any criticism as being a threat because that is what your whole identity relies on. If you're a person who is a Muslim, British and numerous other components making up your identity - you won't feel the need to be concerned about what the other thinks.
But this goes back to the fact that no one knows what it means to be British - who is British? how can you expect a person to integrate into a society when no one knows what it means to be British. You don't have this sort of problem in the US simply because ones national identity is easily accessible and you can bind into the collective zeitgeist with relative ease.