Yes, people are free to demonstrate in UK. Here we have a multi-cultural, multi-religious society (as is the case in many European countries), something that many contributors to this forum despise. It is a society in which different views are tolerated. That makes for a far more peaceful society than one in which different religious groupings feel hatred towards each other.
The photo in question is one in which the end of such peace is advocated. That freedom to call for the end of freedom is a definite problem but so few people subscribe to such a view that it does not theaten the continuation of a peaceful society. We, thankfully, have very, very few religious nutters determined to change the country into a religious state with all the problems that ensue.