The immigration services may differentiate between certain religious groups in order to allow a person such as a minister of religion to enter the UK to provide essential pastoral services, without being challenged by groups which could operate against the public interest, but which might also claim to represent a religion.
A decision to prevent a person who holds extreme religious views from entering or remaining in the country if his or her presence is not conducive to the public good, for example, preachers who use the pulpit to incite violence, would not constitute unlawful discrimination because of religion or belief.