Prevent is one strand of the government’s counter terrorism strategy known as contest. Prevent is the name given to a national strategy which aims to stop people from becoming violent extremists or supporting terrorism. It aims to deal with “all forms of extremism including Far Right racist extremism, animal rights extremism and religious extremism.”
Ecclesiastical Insurance has also produced a Terrorism Prevention Checklist, relevant for your place of worship. The National Counter Terrorism Security Office NaCTSO guidance it references is very useful too.